Using Syntactic Dependencies for Word Alignment

USING SIqVTA CTICDEPENDENCIES FOR WORD ALIGNMENT Fathi DEBILI - Elybs SAMMOUDA - Adnane ZRIBI CNRS- idl 27, rue Damesme, 75013 Paris Phone : (33-1) 43 50 54 01 - Fax : (33-1) 45 89 17 32 e-mall : debili@idl.msh-paris.fr Abstract We attack the problem of aligning words from pairs of bilingual sentences, rather than the well-known, and somewhat easier, problem of aligning sentences. The method that we develop is based on the use of bilingual dictionaries, having supposed that lemmatization has taken place. We first show that this method performs poorly in terms of silence and noise. To improve its performance we introduce syntactic dependency relations between the words in each of the two sentences considered. In this sense the syntagmatic level comes to the rescue of the paradigmatic level at which the alignment actually takes place. I. Introduction Given that two sentences F and E are translations of each other. Is there a simple method for aligning the words in each sentence? In other words is word alignment algorithmically simple to implement? We shall see that this problem is extremely delicate, even to be done by hand. To convince oneself, one need merely attempt it in order to see how quickly the choices pass from trivial to complicated. The difficulties stem from there not always being a simple one-to-one correspondence between the words of the two sentences. One word may correspond to many words (an expression); in other cases, one or many words may correspond to no other words. On the other hand, word order is rarely maintained, and to top things off, different syntactic status create complicated pairings when they do exist. IL Conventions and restrictions concerning manual alignment of words Let's begin by the cases that will be excluded due to the excessive level of difficulty that they present. Consider the two sentences ~ • E17 : BUtl the2 similarities3 are4 illus°rYS" 6 F18 : Ces l c°mparais°ns2 °nt 3 leurs4 limites 5 : 6 1. All the pairs of sentences as examples are extracted from The Acoustics of the Harpsichord (SCIENTIFIC AMER/CAN, February 1991) and its French translation L'acoustique du clavecin (POLrR LA SCIENCE, avril 1991) Although these sentences correspond to each other in the text that they appeared in, we cannot establish an alignment of their words. We will not study these cases for a few reasons: first, outside of their context it is difficult, even for human readers, to affirm their semantic relation as a translation; secondly, in order to align these sentences, the entire sentences must be considered as an expression, and this is debatable. How can manual alignments be represented? We will distinguish the alignment of words and groups of words whose mutual translation is established with the aid of a bilingual dictionary from alignments that are made from a local recomposition based on human "comprehension" of the two sentences. We will use the equal-sign (=) to mark links which come from a bilingual dictionary and the star symbol (*) to mark …

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