The present work investigates whether free inversion is difficult to eliminate from the L2 English grammars of L1 speakers of a null subject Romance language, European Portuguese (EP), and whether L1 interference can be overcome as a result of explicit instruction. The work comprises two interrelated studies: a study on subject placement in L2 English, which was conducted with advanced and near-native learners, using timed and untimed tasks, and a teaching intervention study, which was conducted with intermediate and advanced learners, using a pre-test/post-test design. Results reveal that, typically, free inversion is only fully eliminated from L1 EP – L2 English grammars at a near-native level and that instruction can help learners overcome L1 interference, but its effectiveness depends on learners’ proficiency level.