This study argues that positive second-language (L2) data do not necessarily rule out inappropriate L2 grammar. Rather, L2 learners appear to postulate first-language (L1) grammar as an interim theory about the L2, at least in the initial stages of L2 acquisition. The case where L2 grammar intersects L1 concerning time adverbial clauses was chosen as an object of study. A grammaticality judgement test including correct L2 sentences that are compatible with the L1 (CP), ones that are incompatible with the L1 (ICP), and incorrect L2 sentences (TR) that could occur if L2 learners transfer L1 grammar to L2 was devised and given to native speakers of English who study Japanese as a second language. Results indicate that: (1) the L2 learners accept the TR although there is no positive evidence to allow it; (2) they reject ICP in spite of positive evidence to allow it; and (3) this tendency is remarkable especially in the initial stages of L2 acquisition. (Author/VWL)