ﬂection on the errors and inappropriateness, detected by a diagnostic parser, of the sentences composed by the students ﬁlling the blanks set up in the given contexts and situations. In this paper we report on prototyping the diagnostic parser implemented using LTAG formalism as a component of the system. LTAG(Lexicalized Tree Adjoining Grammar) is a lexicalized grammatical formalism (XTAG Research Group, 1995). For ease of diagnosing the erroneous sentences composed by the students, lexicalized type of grammars seemed most suitable. Comparing HPSG(Head-driven Phrase Structure Grammar) ( et al., 1994) and LTAG, the well-known two (almost-)lexicalized grammars, LTAG looked more.