This study contributes to the debate about the importance of focus on form to adult second language acquisition with an investigation of a Portuguese-speaking adult's acquisition of English present perfect from a historical, correlational, and qualitative viewpoint. Using a husband-wife interaction in the L1, it investigates whether explicit grammar instruction and error correction can lead to automatic production. The focus of the study, the distinction between L1 present simple and L2 present perfect, is contrasted with a control distinction: L1 stative and L2 progressive. The importance of these distinctions is that both are semantically challenging for the L2 acquirer; therefore, they might require focus on form. This study argues against claims based on Krashen's input hypothesis that only comprehensible input can promote acquisition and that explicit data and negative evidence only affect performance. The results confirm the importance of noticing (Schmidt and Frota, 1986) as an essential aspect of adult second language acquisition. With a thorough look at the semantics of the present perfect, the study shows that focus on form is highly recommended for the acquisition of complex structures.