The distinction of two types of intransitive verbs—unergatives (with underlying
subjects) and unaccusatives (with underlying objects)—may not exist at early stages of L2
acquisition, both being syntactically represented as unergatives. This idea, referred to here as the
Unaccusative Trap Hypothesis, provides an elegant developmental account for a variety of
seemingly unrelated syntactic phenomena in L2 English, Japanese, and Chinese. Target language
input, structural constraints on natural language linking rules, and linguistic properties of a
learner's L1s shape stages in the reorganization of the lexical and syntactic components of
interlanguage grammars. Although nonnative grammars may initially override the structural
constraints postulated as the Unaccusative Hypothesis (Burzio, 1986; Perlmutter, 1978) and the
Uniformity of Theta Assignment Hypothesis (Baker, 1988), at later developmental stages some
may still achieve conformity with the norms of natural languages.