A Lnguge-Neutrl Representtion of Temporl Informtion Rihrd Cmpell*, Tkko Aikw, Zixin Jing, Crmen Lozno, Mite Melero nd Andi Wu Mirosoft Reserh One Mirosoft Wy, Redmond, WA 98052 USA {rihmp, tkko, jingz, lozno, mitem, ndiwu}@mirosoft.om *to whom inquiries should e ddressed Astrt We propose frmework for representing semnti tense tht is lnguge-neutrl, in the sense tht it represents wht is expressed y different tenses in different lnguges in shred forml voulry. The proposed frmework llows the representtion to retin surfe distintions for prtiulr lnguges, while llowing fully semnti representtions, suh s representtion of event sequene, to e derived from it. The proposed frmework lso supports the inorportion of semnti tense informtion tht does not derive from grmmtil tense, ut derives insted from other expressions suh s time dverils. The frmework is urrently implemented in NLPWin, multi-lingul, multi-pplition nturl lnguge understnding system urrently under development t Mirosoft Reserh, ut the representtionl frmework is in priniple independent of ny prtiulr system. 1. Introdution 1 Multilingul pplitions fe (t lest) two prolems in the domin of semnti tense: First, there is the prolem tht grmmtil, or morphologil, tenses in different lnguges do not men the sme thing. In English, for exmple, grmmtil pst tense situtes n event prior to the utterne ( speeh time in Reihenh s (1947) terminology), nd grmmtil present tense situtes n event simultneous with the utterne. In ontrst Jpnese pst tense situtes n event prior to some referene time, nd present tense situtes n event simultneous with some referene time, where the referene time my or my not e the utterne time. Neither lnguge hs tense tht expresses extly wht is expressed y pst or present in the other. This poses prolem for pplitions suh s mhine trnsltion (MT), sine given grmmtil tense in one lnguge does not utomtilly trnslte into the sme surfe tense in nother lnguge: (1) 彼 女 は 病 気 だと 言 った knozyo-w [yooki d ] to itt she -Top sik e-pres tht sy-pst she sid [she ws sik] In (1), for exmple, the grmmtil present tense in the emedded luse (indited y rkets) trnsltes into English s grmmtil pst tense, oth of whih llow the interprettion tht the event desried in the emedded luse is simultneous with tht desried in the min luse. Another prolem is tht wht is expressed s grmmtil tense in one lnguge is sometimes only expressile s n dveril onstrution in nother lnguge. For exmple, Chinese hs no grmmtil tense per se (see Setion 3.3 for more detils); onsequently 1 We would like to thnk three nonymous reviewers nd our ollegues in the Nturl Lnguge Proessing group t MSR for their helpful omments nd disussion, espeilly Mihel Gmon, Mris Jimenez, Jessie Pinkhm nd Hismi Suzuki. single form n in priniple express pst, present or future; this is illustrted in the following exmples: (2) 昨 天 他 来 看 我 zuotin t li kn wo yesterdy he ome see me Yesterdy he me to see me. (3) 明 天 他 来 看 我 mingtin t li kn wo tomorrow he ome see me Tomorrow he will ome to see me. In (2) nd (3), the dverils zuotin yesterdy nd mingtin tomorrow re ll tht indite tht these sentenes re set in the pst nd future, respetively. In this pper, we propose frmework for representing semnti tense, y whih we men informtion out the sequene of events. Our frmework is lnguge-neutrl, in the sense tht it represents surfe tense mrking of vrious lnguges using shred forml voulry. Our frmework lso llows the inorportion of semnti tense informtion tht is not expressed s grmmtil tense, for exmple, tht (2) is out pst time. Also, sine lrge prt of wht is expressed y tenses onerns the sequene of events nd sttes, one spet of our frmework is enling n expliit representtion of temporl sequene. The nlyses reported here re urrently implemented in the NLPWin system under development t Mirosoft Reserh (Heidorn, 2000). Most (if not ll) other proposls for lnguge-neutrl representtion of tense, suh s Vn Eynde (1997), re expliit ttempts to represent the semntis of tense diretly. However, the kind of semnti representtion of tense my vry onsiderly depending on pplition. For exmple, some pplitions my require tense to e represented in first-order predite lulus, perhps inorporting Dvisonin event rguments (Dvidson, 1980), while others might require only n expliit sequene of events, s in Filtov nd Hovy (2001). The novelty of our pproh lies in the ft tht it does not ttempt to e prtiulr semnti representtion. Our gol is to preserve syntti informtion out semnti tense so tht vrious semnti representtions of
tense n e onstruted if neessry for prtiulr pplition. For exmple, our representtion is omptile with oth the referentil theory of tense (e.g. Enç, 1987) nd the quntifitionl theory of tense (e.g. Ogihr, 1995). Also, lthough it does not express sequene of events diretly, representtion of suh sequene n e derived from our representtion. Our frmework owes muh to Reihenh (1947); ut while stritly Reihenhin pproh to tense my work well for Europen lnguges, suh n pproh eomes unwieldy when fed with set of lnguges with more typologilly diverse tense systems, inluding Jpnese nd Chinese, spets of whih re disussed elow. We therefore do not rely on the Reihenhin notions of referene nd event times, s does e.g. Vn Eynde (1997), ut dpt wht we tke to e Reihenh s essentil insights to wider rnge of tense systems. 2 Before proeeding, it is neessry to sy something out the terms tense nd spet, nd to ly out wht the sope of the pper is. By semnti tense, we men informtion out how events or situtions re sequened; this inludes some of wht in some trditions is lled spet, suh s the interprettion of the English perfet, et. It lso inludes informtion tht my not e reorded y grmmtil tense, s shown in (2) nd (3). By spet, we men temporl informtion tht goes eyond temporl sequene, suh s (im)perfetivity, progressive, sttive, hitul, nd the like. In this pper, we re onerned with semnti tense, not primrily with spet, though some spetul fetures re onsidered in Setion 3.3.2, elow. The pper is orgnized s follows: In Setion 2 we outline the generl frmework of Lnguge-Neutrl Syntx (LNS) (Cmpell, 2002; Cmpell & Suzuki, 2002), within whih we situte the urrent proposl; in Setion 3, we ly out our proposl for the representtion of semnti tense; in Setion 4, we ompre our system to other proposls for representing semnti tense; Setion 5 offers onlusion. 2. Lnguge-neutrl syntx In this setion we desrie the si properties nd motivtion for LNS. For more detiled desriptions, the reder is referred to Cmpell (2002) nd Cmpell & Suzuki (2002). LNS is level of representtion tht is more strt thn surfe-syntti nlysis, yet not s strt s fully-rtiulted semnti nlysis; rther, it is intermedite etween the two. The si design priniple of LNS is tht it e lose enough to the surfe syntx of individul lnguges to llow reonstrution of the surfe struture of given sentene (i.e., LNS n serve s the input to lnguge-prtiulr genertion funtion), yet strt nd lnguge-independent enough to llow derivtion of deeper semnti representtions, where neessry, y lnguge-independent funtion. The role of LNS is illustrted shemtilly in Figure 1. 2 However, we do use the terms referene time nd event time informlly elow. SURFACE SYNTAX L1 syntx L2 syntx L3 syntx nlysis genertion Lnguge-Neutrl Syntx (LNS) Figure 1: Lnguge-Neutrl Syntx SEMANTICS lexil dependenies logil rep. x[(,)] other semnti rep. The primry motivtion for suh n intermedite representtion is to medite etween lnguges in multilingul pplitions, given tht fully rtiulted semnti representtions re typilly not needed in most suh pplitions. For exmple, the Adjetive + Noun omintions lk t nd legl prolem hve identil surfe strutures, ut very different semntis: the first is interpreted s x[lk(x) & t(x)], i.e., s desriing nything tht is oth t nd lk; the seond, however, does not hve the prllel interprettion s desription of something whih is oth prolem nd legl: rther, it typilly desries prolem hving to do with the lw. To urtely nlyze this distintion would require extensive nd detiled lexil nnottion for djetive senses nd, most likely, for lexilized menings of prtiulr Adj + Noun omintions; suh extensive nnottion, if it is even possile, would mke system tht depends on it very rittle. For most pplitions, however, this semnti differene is immteril, nd the extensive nd rittle nnottion unneessry: for exmple, ll tht we need to know to trnslte these phrses into Frenh ht noir lit. t lk nd proléme legl lit. prolem legl is tht the djetive modifies the noun in some wy. LNS is representtion in whih lk t nd legl prolem hve the sme struture, despite their deep semnti differene, nd in whih lk t nd ht noir hve the sme struture, despite their superfiil syntti differene. An LNS representtion is n nnotted tree, in whih onstituents re unordered, nd linked to their prent y leled rs, the lels orresponding to semntilly motivted grmmtil funtions suh s semnti hed, logil sujet, time, et. The LNS tree is nnotted with semntilly motivted fetures nd reltions expressing long-distne dependenies (suh s inding nd ontrol) nd disourse-oriented funtions (suh s topi nd fous). An exmple (somewht simplified, nd with tense not represented for the time eing) is given elow; this figure represents the LNS for this noun phrse efore the implementtion of the frmework for tense representtion presented elow. (4) the t tht ws seen yesterdy NOMINAL1 (+Def +Sing) _SemHeds--t1 _L_Attri-- (+Pss +Proposition) _SemHeds--see1 _L_Su---_X1 _L_Oj---NOMINAL2 _SemHeds--tht1 _Cntrlr: t1 _L_Time-- yesterdy1
The root node (NOMINAL1) is in the upper left; the dughters of given node re indited y leled rs suh s SemHeds (semnti hed), L_Attri (logil ttriutive modifier), L_Oj (logil ojet), nd the like. In ddition to these ttriutes inditing deep grmmtil reltons, there re other ttriutes whih express dditionl reltions mong nodes in the tree. For exmple, the reltive pronoun NOMINAL2 hs Cntrlr ttriute, whose vlue is t1, nd indites tht t1 is the nteedent of the reltive pronoun. The Cntrlr ttriute is not prt of the LNS tree per se; tht is, the vlue of Cntrlr must e prt of the LNS tree independently of the Cntrlr reltion (in this se, s the semnti hed of NOMINAL1). We refer to ttriutes suh s Cntrlr s non-tree ttriutes. For disply purposes only in this pper, we disply non-tree ttriutes s leled rs, even though they re not prt of the LNS tree per se; they will e displyed slightly differently, however, in tht the vlue of the ttriute is introdued y olon, insted of y dshed line. In this exmple we see lso tht pssives re normlized in terms of their rgument struture, ut the ft tht the reltive luse is pssive is reorded in the feture +Pss on. This reflets si design priniple of LNS: The si struture is normlized for vrition oth within nd mong lnguges, ut surfe distintions (suh s the tive/pssive distintion) re retined s muh s possile. Thus n LNS representtion needs to e lose enough to the surfe syntx to indite meningful distintions, yet strt enough to normlize meningless rosslinguisti vrition. 3. Frmework for semnti tense The LNS representtion of semnti tense must therefore stisfy the following design riteri: (5) Design riteri for LNS representtion of tense:. Eh individul grmmtil tense in eh lnguge is reoverle from LNS.. The expliit sequene of events entiled y sentene is reoverle from LNS y lngugeindependent funtion. Criterion (5)() sys tht we must e le to reonstrut, y distint genertion funtion for eh lnguge, how the semnti tense ws expressed in the surfe form of tht lnguge; this riterion will e stisfied if the LNS representtion is different for eh tense in prtiulr lnguge. Criterion (5)() sys tht we must e le to derive n expliit representtion of the sequene of events from n LNS representtion y mens of lnguge-independent funtion. This riterion will e stisfied if the representtion of eh tense in eh lnguge is truly lnguge-neutrl. In this setion we detil frmework for semnti tense tht meets the design riteri in (5). We egin y giving the detils of the si formlism (whih we will dd to in susequent susetions), followed y disussion of the motivtion nd funtion of its vrious spets. 3.1. Bsi frmework: simple tenses 3.1.1. Tense fetures nd reltions In our proposl eh tensed luse ontins distint Tense node, whih is in the L_Tense ( logil tense ) reltion with the luse, nd whih is speified with semnti tense fetures, representing the mening of eh prtiulr tense, nd ttriutes inditing its reltion to other nodes (inluding other Tense nodes) in the LNS tree. Semnti tense fetures n e either glol or nhorle. 3 The si tense fetures, long with their interprettions, re given in the following tles; Tle I shows the glol fetures, nd Tle II the nhorle ones ( U stnds for the utterne time: speeh time in Reihenhin terms): 4 Feture Mening G_Pst efore U 5 G_NonPst not efore U G_Future fter U Tle I: Glol tense fetures Feture Befor NonBefor Aftr NonAftr Tle II: Anhorle tense fetures Mening efore Anhr if there is one; otherwise efore U not efore Anhr if there is one; otherwise not efore U fter Anhr if there is one; otherwise fter U not fter Anhr if there is one; otherwise not fter U The tense fetures of given Tense node re determined on lnguge-prtiulr sis ording to the interprettion of individul grmmtil tenses. For exmple, the simple pst tense in English is [+G_Pst], the simple present is [+G_NonPst +NonBefor], et. Additionl fetures my turn out, on further nlysis, to e neessry; for exmple, mny lnguges mke grmmtil distintion etween immedite future nd generl future, or etween reent pst nd remote or 3 The distintion etween glol nd nhorle tense fetures is very similr to Comrie s (1985) distintion etween solute nd reltive tenses. We hve dopted the different terminology to emphsize tht the glol/nhorle distintion is for fetures, not for tenses per se, s in Comrie s txonomy. 4 Note tht, given their menings, some pirs of Tense fetures re semntilly inomptile with eh other, nd nnot our on the sme node. For exmple, given Tense nnot e [+G_Pst +G_NonPst]. 5 Stritly speking the mening of the glol tense fetures is to express reltion etween given time t nd glolly speified referene time, G. Coneivly, the vlue of G ould vry, depending on vrious ftors inluding genre, disourse ontext, et. However, we urrently hve no theory s to how G might e set to ny vlue other thn U, so we will ssume throughout tht the glol referene time is lwys the sme s the utterne time.
generl pst. We hve nothing to sy out these speifi ontrsts, however, other thn to note tht the frmework we propose is flexile enough to ommodte new tense fetures, if neessry. A Tense node T will lso under ertin onditions hve non-tree ttriute lled Anhr, whih indites reltion tht T ers to some other Tense node (the vlue of the Anhr ttriute must e nother Tense node). Like other non-tree ttriutes suh s Cntrlr, Anhr should e thought of s n nnottion on the si tree, not s prt of the tree itself; tht is, the vlue of the Anhr ttriute must fit into the LNS tree in some independent wy. A Tense node hs n Anhr ttriute if () it hs nhorle tense fetures; nd () meets ertin struturl onditions. For simple tenses, the struturl ondition tht it must meet to hve n Anhr is tht the luse ontining it is n rgument (i.e., logil sujet or ojet) of nother luse; in this se the vlue of Anhr is the Tense node in the governing luse. In the disussion of ompound tenses elow we will ugment the set of suffiient struturl onditions for hving n Anhr. 6 3.1.2. Pst tense in English nd Jpnese As indited in Tle II, if Tense node with nhorle fetures hs no Anhr, then it is interpreted s if nhored to the utterne time U. This mens tht, for exmple, [+G_Pst] Tense nd n unnhored [+Befor] Tense hve the sme interprettion, ll else eing equl. Consider the following English nd Jpnese sentenes, with the relevnt prts of their LNS struture shown: 7 (6) She ws sik. _SemHeds----sik1 _L_Tense----_Tense1 (+G_Pst) (7) 彼 女 は 病 気 だった knozyo-w yooki dtt she -Top sik e-pst She ws sik. _SemHeds---- 病 気 1 (sik) (+Befor) The English nd Jpnese pst tenses re represented differently euse they re semntilly different, though in these simple exmples tht differene is neutrlized. The English simple pst tense is [+G_Pst], inditing tht it denotes time tht is efore U. The Jpnese simple pst tense on the other hnd is [+Befor], inditing tht it denotes time tht is efore its Anhr. However, in this simple root sentene, there is no Anhr, so it is interpreted s if nhored to U; hene the interprettion is efore U. Thus the design riterion (5)() is met, t lest for these simple ses: simple lnguge-independent funtion would yield the orret sequene e_sik < U for oth these exmples. The semnti differene etween the English nd Jpnese pst tenses omes into ply when the Anhr ttriute is present, whih for simple tenses is in luses tht re rguments of higher luse. Consider the following English nd Jpnese exmples, in whih the tense in question (in oldfe) is in n emedded sentene (indiret speeh), represented in LNS s the logil ojet (L_Oj) of the mtrix luse: (8) She sid she ws sik. _SemHeds--sy1 (+G_Pst) _L_Oj--FORMULA2 _SemHeds--sik1 _L_Tense--_Tense2 (+G_Pst) (9) 彼 女 は 病 気 だったと 言 った knozyo-w yooki dtt to itt she -Top sik e-pst tht sy-pst she sid she ws sik _SemHeds-- 言 う1 (sy) (+Befor) _L_Oj--FORMULA2 _SemHeds-- 病 気 1 (sik) _L_Tense--_Tense2 (+Befor) _Anhr: _Tense1 Sine the emedded tense in (8) is +G_Pst, its interprettion is efore U; left unspeified is whether the sitution desried y the emedded luse (FORMULA2) is reported to hve ourred efore, or simultneous with, the sitution desried y the mtrix luse. In ft, oth interprettions re possile in this se: her reported sikness my e simultneous with her sying tht she ws sik (i.e., she sid I m sik ), or it my hve preeded it (i.e., she sid I ws sik ). 8 The struture we ssign to it ptures tht underspeifition suintly. In (9), on the other hnd, the emedded tense, _Tense2, is +Befor; sine it hs n nhorle feture, nd its luse is the logil ojet of nother luse, it must e nhored to the tense of tht mtrix luse, i.e., to _Tense1. Consequently, it denotes time tht is efore the time denoted y _Tense1 (whih, like _Tense1 in (7), denotes time efore U). So the only interprettion (9) hs is tht her reported sikness is prior to her sying tht she ws sik; i.e., it n only men she sid I ws sik ; it nnot men she sid I m sik. This onstrution illustrtes the essentil differene etween the English nd Jpnese pst tense forms: the former diretly expresses 6 We hve not ruled out the possiility of lngugeprtiulr nhoring onditions, ut so fr hve not enountered ny need for them. 7 In this pper we show only the prts of the LNS neessry to illustrte the tretment of tense; for exmple, we leve out logil sujet, et., unless otherwise neessry. Note lso tht the opul is regulrly omitted from the LNS (see Cmpell, 2002). 8 A third logil possiility, onsistent with the interprettion of G_Pst, is tht her sikness ws in the pst (i.e., efore U), ut fter her sying tht she ws sik; i.e., she sid I will e sik. But this kind of interprettion seems to e universlly disllowed without some kind of irrelis mrking on the luse (suh s modl), nd therefore does not need to e seprtely indited.
reltion to U, while the ltter diretly expresses reltion to some referene time, whih my or my not e U. Exmples suh s (8) nd (9) illustrte preisely why the English nd Jpnese grmmtil pst tenses hve different representtions in the urrent frmework. Suppose for exmple tht the Jpnese pst tense were [+G_Pst] (like the English pst), insted of [+Befor]; then Jpnese (9) should hve the sme rnge of interprettions s English (8), in prtiulr it should e le to serve s desription of n event in whih she sid I m sik i.e., where the time of her eing sik oinides with the time tht she sid she ws sik. As noted, however, this interprettion is not ville for (9), s it is for (8). Our nlysis of the English nd Jpnese pst tenses differs from the pproh tken y e.g. Ogihr (1995), who lims tht English nd Jpnese pst tenses men the sme thing, nd tht differenes suh s tht etween (8) nd (9) elow re due to the optionl pplition in English of rule tht deletes the emedded pst tense from the logil form omponent. Our nlysis gives uniform desription to oth the English nd Jpnese grmmtil pst tenses. It is importnt to note tht there is only one sense of the feture Befor (the sme holds true for ll the nhorle fetures in Tle II), nd hene only one mening for Jpnese pst tense, in our system. This is ruil point whih is esily overlooked: phrsed in stritly Reihenhin terms, we my pper to e sying tht the Jpnese pst tense mens either E<R (if it is nhored) or E<S (if not nhored). But this pperne of i-volism is due, we elieve, to n overly rigid dherene to Reihenh s nottion; our own nottion is more flexile, llowing us to hrterize the Jpnese pst tense s univol, while still retining wht we regrd s Reihenh s essentil insights, nmely tht some tenses relte to U nd others to struturlly determined referene time. 3.1.3. Present tense in English nd Jpnese Another good illustrtion of the differenes etween glol nd nhorle tense fetures is provided y the English nd Jpnese present tenses. As in the se of pst tense, the two tenses reeive the sme interprettion in simple sentenes: (10) She is sik. _SemHeds sik1 (+G_NonPst +NonBefor) (11) 彼 女 は 病 気 だ knozyo-w yooki d she-top sik e-pres She is sik _SemHeds 病 気 1 (sik) (+NonBefor) Sine the English present tense in (10) is [+G_NonPst] (s well s [+NonBefor]; see elow), it must denote time tht is not efore U. The Jpnese present tense is just [+NonBefor], so it denotes time tht is not efore its Anhr; sine it lks n Anhr, in this se, it must denote time tht is not efore U. Consequently (10) nd (11) reeive the sme interprettion. Note tht nothing in these representtions diretly expresses nything out the present : G_NonPst is interpreted s not efore U, ut does not hve to e simultneous with U. This is y design: the English grmmtil present tense llows future interprettion s well s present one, s in We spek tomorrow (see Setion 4, elow). Our ssumption is tht present-time referene is the defult denottion for ny Tense whose fetures nd reltions to other time expressions re onsistent with tht interprettion. Similr omments hold for the Jpnese present tense, whih is [+NonBefor] in our nlysis. As in English, the Jpnese present tense lso llows future-time onstrul (see Setion 3.3.3, elow). As in the se of the pst tenses, the differene etween the English nd Jpnese present tenses shows up when there is n Anhr: (12) She sid she is sik. _SemHeds--sy1 (+G_Pst) _L_Oj--FORMULA2 _SemHeds--sik1 _L_Tense--_Tense2 (+G_NonPst +NonBefor ) _ Anhr: _Tense1 (13) 彼 女 は 病 気 だと 言 った knozyo-w yooki d to itt she -Top sik e-pres tht sy-pst she sid she ws sik _SemHeds-- 言 う1 (sy) (+Befor) _L_Oj--FORMULA2 _SemHeds-- 病 気 1 (sik) _L_Tense--_Tense2 (+NonBefor) _ Anhr: _Tense1 In this se, oth emedded tenses re nhored, sine oth hve the nhorle feture [+NonBefor]. The English present tense is [+G_NonPst], however, so _Tense2 denotes time tht is not efore U; it is lso [+NonBefor], so it lso denotes time tht is not efore the (pst) time denoted y _Tense1. Consequently, the period of her sikness must overlp oth the time of her sying tht she ws sik nd the utterne time U (see lso Note 8); in ft, s Enç (1987) notes, this onstrution hs extly tht mening. This exmple lso illustrtes the ft tht given tense my hve ny olletion of mutully-omptile tense fetures, inluding oth glol nd nhorle ones. In ontrst, the Jpnese exmple (13) (the sme s (1)) does not imply tht the period of her sikness inludes the utterne time; insted, the possiility tht she is still sik t the present moment is left open, unlike (12). In our frmework, this is euse the Jpnese present lks glol tense feture. _Tense2 is [+NonBefor] nd not [+G_NonPst] like (12), so its only requirement is tht it denote time tht is not efore the time denoted y its Anhr, _Tense1. As indited in the gloss, the est English trnsltion of (13) is with the pst tense. Exmples like (12) nd (13) illustrte preisely why the
English nd Jpnese present tenses re to e represented differently. 3.2. Compound tenses One of the gret insights of Reihenh s (1947) nlysis of tense is his tretment of ompound tenses, suh s the English present- nd pst-perfet. In this susetion, we outline our representtion of ompound tenses, whih, despite nottionl differenes, is essentilly Reihenhin. We egin y mking forml distintion etween primry nd seondry tenses, the ltter eing tenses, suh s English hve + pst prtiiple, whih require n Anhr within the sme luse, the former eing ll others. Thus eh lnguge-prtiulr tense must e speified s to its fetures, nd whether it is primry or seondry. Consider the following exmple of the pst perfet in English: (14) He hd rrived. _SemHeds rrive1 (+G_Pst) --_Tense2 (+Befor) _ Anhr: _Tense1 We tret English perfet onstrutions s onsisting of two tenses: seondry tense tht is [+Befor], nhored to primry tense, in this se simple pst (hene [+G_Pst]). There is no prinipled upper limit to the numer of Tense nodes in given luse (though prtiulr grmmrs presumly impose de fto limits), though the following onditions must e met for wellformedness: (1) eh luse hs one nd only one Tense tht is not nhored within the luse (though it my e nhored outside the luse); this is the Tense tht designtes the referene time; nd (2) eh luse hs one nd only one Tense whih is not the Anhr of nother Tense in the sme luse (though it my e the Anhr of nother Tense in nother luse); this is the Tense tht designtes the event time. In (14), the first ondition is stisfied y _Tense1, nd the seond ondition is stisfied y _Tense2. In the simple tense exmples disussed in Setion 3.1, oth onditions re stisfied y the sme Tense node. The dvntges of treting the perfet onstrution s ompound tense, insted of s simple tense, re twofold: (1) it llows us to distinguish English present perfet nd simple pst without dditionl fetures (thus helping to stisfy the design riterion (5)()); nd (2) it ptures the ft tht the perfet onstrution o-ours with every simple tense in English, with the sme interprettion. 3.3. Survey of tenses ross lnguges The frmework desried ove is not theory of tense, in tht it does not uniquely determine representtion for eh grmmtil tense in eh lnguge, ut provides lnguge-neutrl voulry for expressing differenes mong grmmtil tenses ross typologilly diverse lnguges. To implement the frmework in n NLP system, then, we need to hve tul nlyses of speifi tenses. In this setion we provide suh nlyses for severl tenses in severl lnguges. 3.3.1. English The disussion ove gives exmples of the pst, present nd perfet tenses in English nd their omintions. Here we give two more exmples of English grmmtil tenses: the future with will 9 nd the pst with used to. Future: Though n rgument might e mde tht the future with will is tully ompound tense, we tke the simpler route here nd nlyze it s distint primry tense with the feture [+G_Future], s in the following exmple: (15) You will e sik. _SemHeds sik1 (+G_Future) Pst with used to: The pst tense formed with used to, s in he used to work here, like the simple pst tense is [+G_Pst], ut differs from the simple pst not only in spetul properties (not treted here), ut lso in tht it hs the nhorle feture [+Befor]. Consider the following exmple: (16) He sid he used to work here. _SemHeds sy1 (+G_Pst) _L_Oj FORMULA2 _SemHeds work1 _L_Tense--_Tense2 (+G_Pst +Befor) _ Anhr: _Tense1 Sine the emedded _Tense2 is [+Befor], it denotes time tht is not only efore U, ut lso efore the (pst) time denoted y _Tense1. This reflets the ft tht in (16), the time tht he worked here must e efore the time tht he sid he used to work here (ompre to (8), ove); tht is, it n only men tht he sid I used to work here, nd nnot men tht he sid I work here. 3.3.2. Other Europen lnguges Aprt from spetul differenes, the tense systems of Western Europen lnguges suh s Frenh, Germn nd Spnish re very similr to tht of English. The spetul differenes re of ourse importnt, nd must e represented in LNS. Although omplete disussion of spet goes eyond the sope of the present pper, we inlude rief disussion of some diferenes etween English nd Spnish here. One notle differene etween Spnish nd English is tht Spnish hs two distint grmmtil pst tenses, the perfetive, or preterite, nd the imperfetive. The 9 Needless to sy, this is not the only wy to express future-time referene in English. The simple present n sometimes e used, nd there re t lest two other onstrutions tht re future only: e going to + infinitive, nd e out to + infinitive. The ltter onstrution hs different mening from the others (immedite future), nd should e distinguished, perhps with feture. The differene etween will nd e going to is hrd to detet, if it exists t ll, ut in keeping with design riterion (5)() they should e distinguished in some wy.
differene is entirely spetul, nd does not pper to ffet the interprettion of sequene of events per se. Another notle differene etween English nd these other lnguges is tht most of them use the simple grmmtil present tense to refer to n event ongoing t the utterne time, s in the following Spnish exmple: (17) Llueve. rin-pres It s rining. The simple present in English, however, nnot e used this wy; English it rins hs only generi or hitul sense. For oth of these distintions, feture inditing the spetul differene is used; in our system, the relevnt fetures re Disrete nd NonDisrete; the former inditing tht events re viewed in their entirety, the ltter tht events re sudivided into ritrrily smll suintervls. Thus the Spnish preterite is [+Disrete], while the imperfet is unmrked for either of these fetures. Also, the simple present in English is [+Disrete], while the simple present in e.g. Spnish is umrked for this feture. Aside from suh spetul differenes, the most notle tense differene etween Spnish nd English is tht the Spnish present progressive, in ontrst to the simple present, is inomptile with future time referene: (18) Vuelvo mñn. return-1sg tomorrow I return/m returning tomorrow (19) Estoy volviendo (*mñn). e-1sg returning tomorrow I m returning tomorrow. This is hndled y ssigning the present progressive the fetures [+G_NonPst +NonBefor +NonAftr] (in ddition to spetul fetures), whih differs from the simple present in eing [+NonAftr]. In (19) there is no Anhr, so the [+NonAftr] feture dittes tht the time referred to is not fter U; i.e, is not in the future; this ounts for this tense s inomptiility with future time dveril. 3.3.3. Jpnese The disussion ove gives some exmples of the simple pst nd present in Jpnese, nlyzed in our frmework s [+Befor] nd [+NonBefor], respetively. Sine there is no seprte future tense in Jpnese, future time referene is normlly hieved with the simple present tense, s in the following exmple: (20) 明 日 雨 が 降 る shit me-g furu tomorrow rin-nom fll-pres Tomorrow, it will rin. _SemHeds 降 る1 (fll) _L_Time 明 日 1 (tomorrow) (+G_Future) (+NonBefor) The feture [+NonBefor] on _Tense1 is omptile with future time referene, s disussed in Setion 3.1.3, ove. The future, s opposed to present, reding of (20) omes from the presene of the dveril shit tomorrow. In Setion 4, we disuss how semnti tense informtion from dverils is inorported into our frmework. 3.3.4. Chinese Unlike the other lnguges disussed ove, Chinese hs no grmmtil tense. As noted in the introdution vis--vis exmples (2) nd (3), semnti tense, when expressed, is often expressed vi dverils, nd not with grmmtil tense; this is disussed in more detil in Setion 4, elow. However, Chinese does hve limited numer of prtiles, trditionlly referred to s spet mrkers, whih, esides inditing spet, lso indite semnti tense informtion. The spetul mening of these prtiles is eyond the sope of this pper, ut we will disuss few exmples to show how they express semnti tense, nd how tht informtion is represented in our frmework. We disuss here the prtiles le, guo nd jing, s in the following exmples: (21) 他 说 他 买 了 书 t shuo t mi le shu he sy he uy Aspet ook He sys/sid tht he hs/hd ought ooks. _SemHeds-- 说 1 (sy) _L_Oj FORMULA2 _SemHeds-- 买 1 (uy) _L_Tense--_Tense2 (+Befor) _Anhr: _Tense1 (22) 他 说 他 买 过 书 t shuo t mi guo shu he sy he uy Aspet ook He sys/sid tht he hs/hd (one) ought ooks. _SemHeds-- 说 1 (sy) _L_Oj FORMULA2 _SemHeds-- 买 1 (uy) _L_Tense--_Tense2 (+Befor) _Anhr: _Tense1 (23) 他 说 他 将 到 美 国 去 t shuo t jing do meiguo qu he sy heaspet to US go He sys/sid tht he will/would go to the US. _SemHeds-- 说 1 (sy) _L_Oj FORMULA2 _SemHeds-- 买 1 (uy) _L_Tense--_Tense2 (+Aftr) _Anhr: _Tense1 In ll these exmples, the tense of the min luse (_Tense1) hs no fetures; we tke this to e the defult se in Chinese, in whih n unmrked luse n e interpreted s pst, present or future (see the disussion of exmples (2) nd (3) in the Introdution, nd Setion 4,
elow). However, spetul prtiles suh s le, guo nd jing n lso ontriute semnti tense informtion, whih we represent s if it were grmmtil tense. The prtiles le nd guo re oth [+Befor] (their differene is spetul, not represented here); in (21) nd (22), the emedded luse Tense is nhored to the mtrix, inditing tht the uying of ooks took ple efore his sying. In ontrst, jing in (23) is [+Aftr], so this exmple mens tht the going to the US tkes ple fter his sying. 4. Deriving semnti tense from syntti ontext It is often the se tht semnti tense informtion is not represented s grmmtil tense per se, ut n ome, t lest in prt, from dverils or other fetures of the syntti environment. We hve seen tht this is one of the min soures of semnti tense informtion in Chinese; n exmple from English is We spek tomorrow, whih is grmmtilly present tense (hene [+G_NonPst +NonBefor], ut semntilly is unmiguously out the future. To del with this sitution, we propose to ugment the frmework outlined in Setion 3 with n dditionl non-tree ttriute Spfrs, whih indites, for given Tense node, ny other temporl expressions in the luse tht ontriutes to the semnti tense of tht luse. Like Anhr, Spfrs is not prt of the LNS tree per se, ut is n nnottion on the tree. The representtion is given elow: (24) We spek tomorrow. _SemHeds spek1 _L_Time tomorrow1 (+G_Future) (+G_NonPst +NonBefor) _ Spfrs: tomorrow1 _Tense1 hs only the fetures of ny present tense, so the representtion stisfies the first design riterion (5)(); ut its Spfrs is the dver tomorrow1, whih itself hs the feture [+G_Future], sine tomorrow is unmiguously in the future. This reltion indites to the lnguge-independent funtion tht derives the expliit sequentil representtion tht the temporl referene of the luse is to time tht is fter U, thus stisfying the seond design riterion (5)(). Note tht design riterion (5)() is stisfied in nother wy, s well: the struture of (24) is different from the struture of sentene with future tense, whih presumly mkes use of the feture [+G_Future] (see elow); thus the distintion etween the sheduled future (Comrie, 1985) in (24) nd the more si future of We will spek tomorrow is preserved. The need for the Spfrs reltion is muh more prevlent in lnguges tht mke little or no use of grmmtil tense, suh s Chinese. Consider the following exmples: (25) 昨 天 他 来 看 我 zuotin t li kn wo yesterdy he ome see me Yesterdy he me to see me. _SemHeds-- 来 1 (ome) _L_Time-- 昨 天 1 (yesterdy) (+G_Pst) _Spfrs: 昨 天 1 (26) 明 天 他 来 看 我 mingtin t li kn wo tomorrow he ome see me Tomorrow he will ome to see me. _SemHeds-- 来 1 (ome) _L_Time-- 明 天 1 (tomorrow) (+G_Future) _Spfrs: 明 天 1 The Spfrs reltion thus permits speifition of semnti tense fetures tht re not expressed s grmmtil tense. 5. Comprison to other frmeworks Our proposl is for system of representtion of semnti tense tht is lnguge-neutrl; i.e., tht represents the tense distintions of different lnguges in forml voulry tht hs the sme mening in ll lnguges. As suh, our proposl is very different from proposls to represent the semntis of tense in prtiulr lnguge suh s English, oth in the ovious respet tht we onsider other lnguges, nd in the less ovious respet tht our proposl is not semnti one in ny deep sense, ut rther syntti representtion tht is lnguge-neutrl, s skethed in Setion 2 (Cmpell & Suzuki, 2002). As suh, the nerest thing to omprle proposl tht we hve enountered in the omputtionl literture is Vn Eynde (1997), whih expliitly provides Reihenhin semnti frmework for multiple lnguges, nd inorportes informtion from temporl dvers in ddition to grmmtil tense. Unlike our proposl, however, Vn Eynde s frmework is expliitly Reihenhin, hrterizing tenses in terms of three possile vlues for stense, expressing the reltion etween the referene nd speeh times, nd six vlues for saspect, expressing the reltion etween the event nd referene times. Although our frmework enodes the sme essentil insight, it does so without rigidly dhering to the referene time/event time distintion, whih leds to simpler representtion in our view. 6. Applition Hving lnguge-neutrl representtion of semnti tense hs ler implitions for multi-lingul pplitions suh s MT. Consider gin the Jpnese exmple (13), in whih n emedded present tense is to e trnslted into pst tense in English. A simple trnsfer of the lnguge-prtiulr present tense yields the wrong result, sine She sid she is sik (=(12)) mens something very different from (13). Insted, wht needs to e trnsferred is the whole temporl struture of _Tense2, inluding its fetures nd its Anhr, sine this is the informtion tht
determines tht it denotes time tht is efore U. Suh ontext-sensitive trnsfers re possile in n MT system suh s tht desried y Rihrdson, et l. (2001). Similrly, onsider the Chinese exmple (25), in whih there is no grmmtil tense speified. A Chinese- English MT system must trnsfer not the grmmtil tense (whih yields no informtion whtsoever), ut rther the whole temporl struture, whih in this se inludes its Spfrs, in order to give the English genertion system the informtion it needs to generte pst tense. 7. Conlusion We hve presented nd exemplified frmework for representing semnti tense in lnguge-neutrl fshion, whih meets the ompeting design riteri in (5): tht eh lnguge-prtiulr tense e reonstrutile y genertion funtion, nd tht n expliit representtion of temporl sequene e derivle y mens of lngugeindependent funtion. The frmework we hve proposed llows us to get semnti tense informtion from grmmtil tense, or from dveril modifiers, nd represents this informtion in semntilly motivted, lnguge-neutrl fshion. 8. Referenes Cmpell, R., 2002. Lnguge-neutrl syntx. MSR Teh Report (in preprtion). Cmpell, R. & H. Suzuki. 2002. A lnguge-neutrl representtion of syntti struture. SCANALU-2002. Comrie, B., 1985. Tense. Cmridge University Press. Dvidson, D. 1980. The logil form of tion sentenes. In D. Dvidson, ed., Essys on tions nd events, 105-122. Clrendon Press, Oxford. Enç, M., 1987. Anhoring onditions on Tense. Linguisti Inquiry 18, 633-657. Filtov, E. & E. Hovy. 2001. Assigning time-stmps to event-luses. In Proeedings of ACL-EACL. Heidorn, G.E. 2000. Intelligent writing ssistne. In R. Dle, H. Moisl nd H. Somers, eds., Hndook of nturl lnguge proessing. Mrel Dekker, New York. Ogihr, T. 1995. The Semntis of Tense in Emedded Cluses. Linguisti inquiry 26, 663-679. Reihenh, H. 1947. Elements of symoli logi. The Free Press, New York, nd Collier-Mmilln, London. Rihrdson S., W. Doln, A. Menezes nd J. Pinkhm. 2001. Ahieving ommeril-qulity trnsltion with exmple-sed methods. In Proeedings of the VIIIth MT summit, Sntigo de Compostel, Spin. 293-298. Vn Eynde, F. 1997. Mood, Tense & Aspet. In F. Vn Eynde nd P. Shmidt (eds.), Linguisti speifitions for typed feture struture frmeworks. EU Commission, Luxemourg.