This study examined the extent to which language-switching practice enhanced L2 learners’ L2 grammatical processing by improving language selection abilities. Thirty-six English-speaking learners of French completed the same language-switching practice of L1 and L2 sentences, but received different types of pre-practice explicit information (EI) designed to address L2 learning difficulties resulting from crosslinguistic influence: one group (n = 17) received EI about English–French differences for viewpoint aspect, and a second group (n = 19) received EI about viewpoint aspect in French only. This design investigated the extent to which pre-practice linguistic knowledge moderated the effectiveness of the language-switching practice. Longitudinal analyses showed that increasing amounts of practice improved language selection abilities (increased accuracy, reduced reaction time costs), but only for learners who received EI about L1-L2 differences. These findings that language-switching was moderated by type of pre-practice EI have important implications for theories of L2 learning and instruction.