Recently ELT has witnessed the debate between the proponents of non-interventionist and interventionist views towards SLA. Different versions of form-focused instruction (FFI) are being practiced despite the fashionable meaning-focused instruction (MFI) of the communicative era. Nevertheless, a question still remains: "when is the time to intervene?" The present research targeting this very question compared the efficacy of proactive and reactive FOF on the grammar improvement of 120 Iranian EFL learners at two proficiency levels of beginner and upper-intermediate. The results indicated that...