Siegel Melanie

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Siegel, Melanie

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Language, Information and Computation(PACLIC 11),1996, 43-52Preferences and Defaults for Definiteness and Numberin Japanese to German Machine TranslationMelanie SiegelDFKI GmbHStuhlsatzenhausweg 3, 66123 Saarbriickensiegel@dfici.uni -sb.dete1.0681-302-5284fax 0681-302-5338AbstractA significant problem when translating Japanese dialogues into Ger-man is the missing information on number and definiteness in the Japaneseanalysis output. The integration of the search for such information into thetransfer process provides an efficient solution. General transfer rules, prefer-ence rules and default rules are combined. The transfer includes conditionsto make it possible to consider external knowledge. Thereby, grammaticaland lexical knowledge of the source language, knowledge of lexical restric-tions on the target language, domain knowledge and discourse knowledgeare accessible.1 IntroductionOne of the significant problems in Japanese to German machine translation isthat information on definiteness and number is in most cases not available onthe surface of the Japanese utterance. Japanese has neither number agreementbetween verbs and nouns nor obligatory plural morphemes. Optional pluralmorphemes like tachi or domo are only available for nouns that refer to persons.However, for the generation of German utterances the generator needs such infor-mation, as in many cases determiners are obligatory. We analyzed the problembased on empirical material collected in the domain of appointment scheduling.In our setting, an interpreter translated 10 dialogues of Japanese and Germanspeakers. Consider the following example from our data:Japanese:kayoobi wa	 watashidomo no	 tokoro deTuesday TOPIC we	 GEN side	 CASE DE43wa	 kyuujitsu na no	 de tabun kaigiTOPIC holiday	 copula	 maybe meetingni	 sanka	 suru koto wa	 dekimasenCASE NI participate do	 NOM TOPIC cannotGerman translation:auf unserer Seite ist Freitag ein Feiertag vielleichton our	 side is Friday a	 holiday maybekOnnen wir an dem Treffen nicht teilnehmencan	 we at the meeting not	 participate(On our side Friday is a holiday and we maybe cannot participate in the meeting)The information that Feiertag has to be preceded by the singular indefinite(masculine) determiner ein and that Treffen has to be preceded by the singulardefinite (neuter) determiner dem does not come out of the surface of the Japaneseutterance and therefore cannot be included in the parsing result. It is not anadequate solution to transfer an underspecified representation to the Germangeneration module, because the information that is needed to decide on thedefiniteness and number of the noun phrase partly comes out of the Japanesesurface, partly out of German lexical restrictions and partly out of domain anddiscourse restrictions. Not all of this information is available in the generationphase. We argue that it is an interlingual problem and therefore must be solvedin the transfer module.2 Previous approaches[Murata and Nagao, 1993] describe a solution model that searches for infor-mation at the surface of the Japanese utterances to give hints for the choiceof determiners in the English counterpart. They state heuristics for the pos-sibility of definiteness and number values concerning words or constructions inthe Japanese utterances. Such information can be determiners in Japanese orparticle and tense information.But this is only one of the relevant aspects, because it does neither considerrestrictions on definiteness and number coming from the target language, norrestrictions based on domain and discourse. Just as little as it is an inherentGerman problem it is an inherent Japanese problem.[Bond et al., 1994] already state that the inclusion of information about thetarget language (English in their case) increases the rate of correct translations.44They enrich the heuristics with information on countability of English nouns.But their approach still lacks the integration of knowledge about discourse anddomain, which is relevant as we will show.Every approach that is external to the transfer process cannot include sourceand target language information at the same time and be thus effective. It isessential to find a mechanism that makes it possible to consider discourse anddomain knowledge.3 Transfer rulesOnly in some cases, where definite articles, quantifying adjectives or certaingenitive constructions can be found in the Japanese source language utterance,the parsing result can directly contain information on number and definiteness.The information that is necessary to generate the German determiners has to besearched in the surface of the Japanese utterance (as it is described by [Murataand Nagao, 1993]), in lexical information of the German target language (as itis described by [Bond et al., 1994]) and in external knowledge sources.The solution of our approach is based on the idea of transfer-based machinetranslation. The representation for the transfer rules are expressions in simplifiedRQLF-format [Aishawi, 1992]. An RQLF, Resolved Quasi-Logical Form, is anunderspecified semantic representation that includes context information. Nounswith determiners are represented as qterms. An example is the representationfor a house:qterm = (< t quant, p = inde f , n = sing, 1 = a >, house)t is the type of expression, p the phrase type, n contains number informationand 1 the lexical realization.The integration of the search for definiteness and number in the transfer processreduces the complexity of the problem, because it is possible to state a numberof transfer rules without searching for information on number and definiteness.Another advantage is that no extra process has to be activated to find informationon the Japanese sentence surface. A practical aspect is therefore the avoidanceof redundancy in the translation process, contrasting Murata/Nagaos approach.Only when no transfer rule can be found that directly gives information ondefiniteness and number, preference rules are activated to search for the missinginformation. The rule format of the transfer rules is the following:transfer(JapaneseRQLF	 GermanRQLF) : conditions.It contains the Prolog predicate transfer, a translation rule expressing the re-lation between the source and target language expression and (optionally) oneor more conditions. The RQLF expressions can be complex, including for exam-ple the representation of a determiner and the corresponding noun. They can45include variables. Conditions are optional Prolog clauses. They can restrict thetransfer rule to a certain value in the RQLF or to a speechact. This makes itpossible to include domain knowledge. They can also be transfer-predicatesso that the rule is recursive. This is needed for the case that a rule inserts in-formation on only number or definiteness and another one is needed to insertthe missing information. Another possibility is the condition definiteness thatlooks for further information on definiteness after the information on numberwas found. These conditions are responsible for discourse knowledge. A combi-nation of conditions is also allowed. The searching strategy of the transfer rulesis determined by the Prolog mechanism of backtracking.3.1 Rules that avoid the necessity to insert information on num-ber and definitenessIn many cases a preferred German equivalent does not contain a noun, andtherefore no more information on number and definiteness is needed. These are— on the one hand — general translation equivalents for complex expressionsand — on the other hand — realizations of speechacts in the domain. Examplesfor the first ones are:• hayai jikan - friih (early time - early)• nagai jikan lange (long time - long)• yasumi - geschlossen (holiday - closed)Such general translation equivalents are easy to state, as for example nagai jikan- lange:transfer([jikanl,nagail]	[lang_ prop]).Temporal expressions are translated stereotypically without searching for infor-mation on number and definiteness, as for example niji ni um zwei Uhr (at oneo'clock). Some Japanese noun phrases that contain two nouns connected by agenitive no can have German equivalents that contain only one noun: watashido-mo no tokoro - wir (our side - we). Other Japanese noun phrases containingno-phrases have a German equivalent with a nominal compound: getsuyoobi nogogo - Montag Nachmittag (afternoon of Monday - Monday afternoon). TheGerman nominal compound gets only one determiner; as soon as a restrictionfor one of the parts is found, the determiner can be decided on. Other casesare domain-specific: Japanese mina in our data is always translated as alle Mit-arbeiter (all staff-members), but could be - for example in another domain -alle Studenten (all students). It always (independent of the domain) has to betranslated with plural. The rule in our domain is:transfer([mina][qterm = [t = quant, p = de f, n = plural, l alle], Mitarbeiter]) .46An example, where the number information is dependent on the domain is thefollowing:Japanese:kono	 hi	 wa	 sham	 wa	 kimasenthis	 day TOP staff-member TOP do not comeGerman:An	 diesem	 Tag	 kommen	 die	 Mitarbeiteron	 this	 day	 come,pl.	 the,pl.	 staff membersnichtnotor:An	 diesem	 Tag	 kommt	 der	 Mitarbeiteron	 this	 day	 come,sg	 the,sg.	 staff membersnichtnotThe translation of sham depends on if the company has one or more staff-members. It could be thought of adding a condition domain(D), if the rulesare used in a system that concerns more than one domain.Strictly speaking, information on gender has to be found, too. But this is aninherent German problem and underlies German lexical restrictions and domainrestrictions and is therefore left to generation. Examples for speechact realiza-tions are yotei wo tatetai - schlage ich vor (I would like to set up the plan -I propose) and donna yotei ni naru ka - wie ist...? (what plan will become- how about...?). By using indicators for speechacts one can try to find prag-matic translation equivalents instead of literal ones: Both examples belong tothe speechact proposal. Thus, transfer rules can include a condition that is aninquiry to the dialogue component. The project VERBMOBIL defines dialogueacts for the appointment scheduling domain, which could also be used for thispurpose. See [Jekat et al., 1995] for further information.The empirical base is built by 10 collected dialogues of appointment schedulingbetween German and Japanese speakers that were translated by an interpreter.The data includes 566 noun tokens. 18.9% of these Japanese nouns have Ger-man equivalents that are not nouns. 34.63% are temporal expressions that aretranslated stereotypically. That makes more than 50% of all nouns where nei-ther search for information on number nor for such on definiteness is necessarywhen first adequate transfer rules — general ones or domain specific ones — aresearched for. This already is a strong argument to integrate the solution of theproblem into the transfer process and to adapt the transfer to the domain.473.2 General rulesNumerals in Japanese give clear information on number and have an unambigu-ous German translation, as for example:• ichijikan, sanjikan - eine Stunde, drei Stunden (one hour, three hours)• hitori, futari - eine Person, zwei Personen (one person, two persons)• hitori no hito, yonin membaa - eine Person, vier Mitglieder (one person,four members)• kenkyuuin no hitori - einer unserer Forscher (one of our researchers)These cases underly general transfer rules for number. Still, information ondefiniteness has to be found, as the following transfer rule shows:transferasanj ikan][qterm = [t = quant, p P, n = plural, 1 = drei], Stunden]) : -definiteness (sanjikan, P).It translates sanjikan into drei Stunden or die drei Stunden. The conditiondefiniteness is a predicate to test whether an entity is pre-mentioned (thatis, included on a stack of pre-mentioned entities) and thus definite, or not pre-mentioned and thus indefinite. In our implementation, the stack is not only usedfor determining definiteness, but also for resolving zero pronominals (see [Siegel,1996] for further information). These cases concern 7.42% of the nouns in ourdata base.In some cases Japanese nominal phrases contain determiners, as kono jikantai(this period of time/these periods of time) and sono jikan (that time/that times).In these cases the translation concerning definiteness is straightforward:transfer([qterm = [t = quant, p = de f , n = N, 1 = kono], X][qterm = [t = quant, p = de f, n = ND, 1 = dies], X DD : -transfer (X,XD).A definite (p = de f) Japanese nominal phrase with a determiner kono and with-out information on number (n = N) is transferred to a definite German nominalphrase with a determiner dies (this) and German number information ND. Thecall trans f er(X , XD) initiates the search for a transfer equivalent of the nounphrase and its number information and unifies the result with the found qterm.But not only determiners lead to a situation where transfer rules concerning def-initeness can be stated straightforwardly. Other possibilities are some kinds ofadjectives and genitive constructions, as for example:48• onaji shun - dieselbe Woche/dieselben Wochen (the same week(s))• tsugi no hi - der ndchste Tag/die niichsten Tage (the next day(s))• kondo no kaigi - das nachste Treffen/die niichsten Treffen (the next meet-ing(s))All of these require a translation with definite determiner.3.3 Preference rules and the default[Schmitz and Quantz, 1993] present an hybrid model for the search for informa-tion that combines exact knowledge with default knowledge. Default knowledgecan be formulated as valid in a domain. Some entities in a domain are knownand unique. These have to be translated singular and definite. Those are in ourdomain, for example, days of the week, the lunch break and the meeting. It isnecessary to include domain-specific default transfer rules, as for example:transfer(kaisha[qterm	 = quant , p = de f, n = sg , 1 = L], Firma]) .Pre-mentioned entities have to be kept in a stack and have to be translated withdefinite determiner. An option with strong preference is to copy the informationon number from the previous mentioned entity, as in the following example:Japanese:kono	 jikantai	 wa	 dekireba	 saketethis	 period(s) of time	 TOP	 if possible	 keep freeitadakitai	 to	 omoimasuHON	 COMPL	 thinkGerman:Ich	 denke,	 daf3	 ichI	 think	 thatdiese Zeitriume	 mOglichstthese periods of time	 if possiblediesen Zeitraumthis period of timefreihalten	 mOchtekeep free	 wantThe translation depends on if it was spoken about one or more periods of timebefore.In German sentences with copula predicates number agreement between subjectand x-complement is preferred. This can be stated as a preference rule. Thisis not a general rule, as the example wir sind ein Projektteam (we are a projectteam) shows. Consider the following examples:49Japanese:getsuyoobi	 wa	 kyuujitsu	 desuMonday	 TOP	 holiday	 isGerman:Montag	 ist	 ein	 FeiertagMonday	 is	 a	 holidayandJapanese:getsuyoobi	 to	 kayoobi	 wa	 kyuujitsu	 desuMonday	 and Tuesday TOP	 holiday	 isGerman:Montag	 and	 Dienstag	 sind	 FeiertageMonday	 and	 Tuesday	 are	 holidaysThe transfer rule is as follows:transfer([desu,QTERMJ,ARG1J,ARG2Jj[sein,QTERMD,ARG1D,[qterm = [t quant, p = inde f , n = N, 1 = L]], ARG2D]) : —transfer(ARG1J,ARG1D), 	 %transfer the first argumentvalue(qterm/n,ARG1D,N), 	 %copy information on numbertransfer(QTERMJ,QTERMD).If no transfer rule or preference rule is applicable, singular indefinite is insertedas a default. The analysis of the data shows that in most cases that do not fallunder the categories described above, this default leads to a correct translation.The preferences for number are:1. Number information that comes from the Japanese nouns is copied intothe German qterm.2. Known and unique entities in the domain are translated with singulardeterminer.3. Number agreement in copula sentences.4. Default: Singular.The preferences for definiteness are:501. Known and unique entities in the domain are translated with definite de-terminer.2. Noun phrases with kono, sono, dono, onaji no, tsugi no, or kondo no in anominal phrase are translated definite.3. •Pre-mentioned entities are translated with definite article.4. Default: Indefinite.4 SummaryThe problem of missing number and definiteness in translating Japanese nounsinto German is significant as it occurs with every Japanese utterance that hasto be translated. To solute the problem it is necessary to combine knowledgeon the Japanese source language with such on the German target language anddiscourse and domain knowledge. Previous approaches lack this possibility toconnect the different knowledge sources.We integrate preference rules and default rules concerning number and defi-niteness into the transfer and are therefore able to consider source and targetlanguage at the same time. Domain and discourse knowledge can be referredto by conditions that restrict the transfer rules. Preference rules are stated fordomain-specific and discourse knowledge. Domain-specific knowledge is encodedin a stack of unique entities of a domain. Discourse knowledge is also encoded ina stack on pre-mentioned entities. The following picture shows the connection ofdifferent kinds of transfer rules to translate Japanese to German noun phrases.discourse-specifictransfer rules general transferrules             domain-specifictransfer rules transfer of nouns  transfer withconditions onspeechacts                    preference rules                      general defaultrules  domain-specificdefault rules         We have shown that combinating transfer and the search for information onnumber and definiteness reduces the problem to a reasonable extend. It can beshown that though general rules have to be preferred to domain-specific ones,51domain-specific rules play an important role in translating Japanese noun phrasesinto German.The described transfer rules and preference restrictions are based on observationson a corpus. They are implemented in a Prolog program.References[Alshawi, 1992] H. Alshawi. The Core Language Engine. Cambridge: The MITPress, 1992.[Bond et al., 1994] F. Bond, K. Ogura, and S. Ikehara. Countability and numberin Japanese to English machine translation. In Proceedings of Coling '94, pages32-38, 1994.[Jekat et al., 1995] Susanne Jekat, Alexandra Klein, Elisabeth Maier, IlonaMaleck, Marion Mast, and J.Joachim Quantz. Dialogue acts in VERBMOBIL.Verbmobil-Report 65, Universitat Hamburg, DFKI Saarbriicken, UniversitatErlangen, TU Berlin, 1995.[Murata and Nagao, 1993] M. Murata and M. Nagao. Determination of referen-tial property and number of nouns in Japanese sentences for machine trans-lation into English. In Proceedings of the 5th International Conference onTheoretical and Methodological Issues in Machine Translation, pages 218-225,1993.[Schmitz and Quantz, 1993] B. Schmitz and J. Quantz. Defaults in machinetranslation. KIT-Report 106, Technische Universitat Berlin, 1993.[Siegel, 1996] Melanie Siegel. Die maschinelle Ubersetzung aufgabenorien-tierter japanisch-deutscher Dialoge. LOsungen fur Translation Mismatches.Ph.D.Thesis, Universitat Bielefeld, 1996.52

Preferences and Defaults for Definiteness and Number in Japanese to German Machine Translation

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Preferences and Defaults for Definiteness and Number in Japanese to German Machine Translation

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