Interpreting Syntactically Ill-Formed Sentences

The paper discusses three different kinds of syntactic ill-formedness: ellipsis, conjunctions, and actual syntactic errors. It is shown how a new grammatical formalism, based on a two-level repr_e sentation of the syntactic knowledge is used to cope with Ill-formed sentences. The basic control struc ture of the parser is briefly sketched; the paper shows that it can be applied without any substan tial change both to correct and to ill-formed sen tences. This is achieved by introducing a mechanism for the hypothesization of syntactic structures, which is largely independent of the rules defining the well-formedness. On the contrary, the second level of syntactic knowledge embodies those rules and is used to validate the hypotheses emitted by the first level. Alternative hypotheses are obtain ed, when needed, by means of local reorganizations of the parse tree. Sentence fragments are handled by the same mechanism, but in this case the second level rules are used to detect the absence of one (or more) constituents.