This study examined how key factors of FFI (degree of explicitness, L1 similarity, proficiency level, and grammar type) influence the accuracy of production. A total of 22 experimental studies, all of which had Korean participants and productive assessments of speech or writing, were selected for analysis. Results revealed that explicit grammar emphasis was more effective for morphology that had a small, binary scope (present or absent). Results further revealed that explicit emphasis of grammatical features dissimilar from the L1 significantly increased accuracy of learner speech and writing. In contrast, implicit emphasis was effective with grammatical features that had a larger scope (e.g., many lexical forms or syntactic arrangements), as well as with grammatical features that were similar to the L1. Findings suggest that explicit emphasis of a smaller scope and implicit emphasis of a larger scope are both useful, since they do not cognitively overload the learner during communication.