Grammar varies in semantic, morphosyntactic, and phonological complexity, which may influence what type of Corrective Feedback (CF) is effective. The present study was designed to investigate the best form of CF for each grammatical feature, in conjunction with associated variables such as learner proficiency level and L1 influence. Fifteen studies, all of which had English as a Foreign Language (EFL) learners with a Korean L1 and productive measures of speech or writing, were selected for study. Results suggest that grammatical features which are very similar to the L1 may benefit more from implicit reformulations (recasts). Explicit CF (direct and metalinguistic feedback) appears more useful for semantically and syntactically complex features such as the English article and past hypothetical conditional. Finally, research of proficiency level suggests that timely emphasis of specific grammatical features is needed in order for CF to be effective.