A Proper Treatmemt of Syntax and Semantics in Machine Translation

A PROPER TREATMEMT OF SYNTAX AND SEMANTICS IN MACHINE TRANSLATION ¥oshihiko Nitta, Atsushi Okajima, Hiroyuki Kaji, Youichi Hidano, Koichiro Ishihara Systems Development Laboratory, Hitachi, Ltd. 1099 Ohzenji Asao-ku, Kawasaki-shi, 215 JAPAN ABSTRACT A proper treatment of syntax and semantics in machine translation is introduced and discussed from the empirical viewpoint. For English- Japanese machine translation, the syntax directed approach is effective where the Heuristic Parsing Model (HPM) and the Syntactic Role System play important roles. For Japanese-English translation, the semantics directed approach is powerful where the Conceptual Dependency Diagram (CDD) and the Augmented Case Marker System (which is a kind of Semantic Role System) play essential roles. Some examples of the difference between Japanese sentence structure and English sentence structure, which is vital to machine translation~ are also discussed together with various interesting ambiguities. I INTRODUCTION We have been studying machine translation between Japanese and English for several years. Experiences gained in systems development and in linguistic data investigation suggest that the essential point in constructing a practical machine translation system is in the appropriate blending of syntax directed processing and the semantics directed processing. In order to clarify the above-mentioned suggestion, let us compare the characteristics of the syntax directed approach with those of the semantics directed approach. The advantages of the syntax directed approach are as follows: (i) It is not so difficult to construct the necessary linguistic data for syntax directed processors because the majority of these data can be reconstructed from already established and well-structured lexical items such as verb pattern codes and parts of speech codes, which are overflowingly abundant in popular lexicons. (2) The total number of grammatical rules necessary for syntactic processing usually stays within a controllable range. (3) The essential aspects of syntactic processing are already well-known, apart from efficiency problems. The disadvantage of the syntax directed approach is its insufficient ability to resolve various ambiguities inherent in natural languages. On the other hand, the advantages of the semantics directed approach are as follows: (i) The meaning of sentences or texts can be grasped in a unified form without being affected by the syntactic variety. (2) Semantic representation can play a pivotal role for language transformation and can provide a basis for constructing a transparent machine translation system, because semantic representa- tion is fairly independent of the differences in language classes. (3) Consequently, semantics directed internal representation can produce accurate translations. The disadvantages of the semantics directed approach are as follows: (I) It is not easy to construct a semantic lexicon which covers real world phenomena of a reasonably wide range. The main reason for this difficulty is that a well-established and widely-accepted method of describing semantics does not exist. (For strongly restricted statements or topics, of course, there exist well-elaborated methods such as Montague grammar [2], Script and MOP (Memory Organization Packet) theory [13], Procedural Semantics [14], and Semantic Interlingual Representation [15].) (2) The second but intractable problem is that, even if you could devise a fairly acceptable method to describe …

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