It remains unclear whether adult-learned second language (L2) depends on similar or different neurocognitive mechanisms as those involved in first language (L1). We examined whether English past tense forms are computed similarly or differently by L1 and L2 English speakers, and what factors might affect this: regularity (regular vs. irregular verbs), length of L2 exposure (length of residence), age of L2 acquisition (age of arrival), L2 learners’ native language (Chinese vs. Spanish), and sex (male vs. female). Past tense frequency effects were used to examine the type of computation (composition vs. storage/retrieval). The results suggest that irregular past tenses are always stored. Regular past tenses, however, are either composed or stored, as a function of various factors: both sexes store regulars in L2, but only females in L1; greater lengths of residence lead to less dependence on storage, but only in females; higher adult ages of arrival lead to more reliance on storage. The findings suggest that inflected forms can rely on either the same or different mechanisms in L2 as they do in L1, and that this varies as a function of multiple interacting factors.