Korean-English MT and S-TAG

An earJy motivation for Synchronous TAG (S-TAG) (Shieber and Schabes, 1990) was machine translation (Abeille, Schabes and Joshi, 1990). Abeille et al note that traditionally difficult problems outlined by Dorr (1994}-for example, categorial, thematic, conflational, structural and lexical divergences-have been used to argue for the necessity of an explicit semantic representation. However, many of these divergences are not probIems for an S-TAG-based approach. Synchronous TAG translation models thus allow us to explore the question of the extent to which a semantic representation is actually necessary. S-TAG was redefined by Shieber (1994) for both theoretical and practical reasons, introducing the requirement that the derivation trees of target and source be isomorphic. Under this definition it has been noted (Shieber, 1994; Dras and Bleam, 2000) that there are mappings that cannot be described under S-TAG. This was the motivation for meta-level grammars (Dras, 1999), by which two TAG grammars can be paired while retaining their original properties, as under standard S-TAG, allowing for a description of mappings that include unbounded non-isomorphisms (Dras and Bleam, 2000). This work on exploring how S-TAG (with and without meta-level grammars) can be used for MT has only been applied to languages that are closely related-English, French,"Italian and Spanish. In this paper we aim to take a much more widely differing pair of languages, English anq 'Korean, to investigate the extent to which syntactic mappings are satisfactory. English and Korean have a wide range of differences: rigid SVO word order in English vs verb-final with free word order in Korean, the largely analytic structure ofEnglish vs the agglutinative structure ofKorean with its complex morphology, optional subject and object and the absence ofnumber and articles in Korean, and many . others. These all suggest hat a meta-level grammar will be necessary as there are various many-to-one or n:ianyto-many mappings between derivation セ・@ nodes (i.e., there will be few cases where a single elementary tree corresponds to another single elementary tree, which has been the case with closely related languages) . Although there is an implemented Korean/English MT system that includes a TAG Korean parser as a source language analysis component (Han et al., 2000), this system as a whole is based on Meaning Text Theory (Mel' cuk, 1988), an enriched dependency fonnalism. Thus, it requires a conversion component that converts the TAG parser output to a dependency notation. As pointed out in Palmer et al. (2002), however, this conversion process resulted in a loss of crucial infonnation such as predicate-argument s ructure encoded in TAG elementary trees, which had negative consequences in the translation results. This then provides further motivation to explore the feasibility of applying a single TAG-based formalism to modeling and implementing a Korean/English MT system. As a first step towards exploring the extent to which an S-TAG style approach can successfully model these widely different languages, we have taken from a parallel English-Korean Treebank twenty examples of divergent constructions (see Appendix). Each halfhas roughly 50,000 word tokens and 5,000 sentences. While the annotation guidelines for the Korean half was developed in Han, Han and Ko (2001) for this corpus, the Englisb half follows the guidelines already developed for Penn English Treebank (Bieset al., 1995), as closely as possible. The example pairs represent structures including copula, predicative/attributive adjective, passive, causative, inter· rogative, relative clause, complex verb, and modal construction, among others. We find that using a TAG-based meta-level gramrnar to model Korean/English correspondences for machine translation is quite feasible.