Two studies on L2 acquisition of the progressive marker zai in Mandarin Chinese by native English speakers were conducted to investigate the interaction between L1 influence and the congruence of lexical aspect and tense-aspect morphology, as formulated in the aspect hypothesis. The two factors make opposite predictions with respect to the early stage and the acquisition process. The findings from a judgment task and a production task show that the observed pattern is neither predicted by the aspect hypothesis alone nor entirely conditioned by L1 influence. Rather, it is the result of both forces at work. At the early stage zai is associated with activities and accomplishments involving goal or distance. In the acquisition process, both widening and narrowing of predicate types are observed. The findings also show that the L1 effect does not disappear at the same time, but proceeds in stages. In the case of zai marking, the L1 effect weakening process is governed by the strength of event ending that is part of the meaning of the predicates.