The study investigates which grammar features need high priority of pedagogical intervention to develop implicit knowledge, unlike grammatical features in need of explanation of explicit rules. Two types of data were collected from 29 Korean college students: a sentence completion test in English and a meta-linguistic knowledge test (requiring students to both correct the underlined error and explain the grammatical rule) in Korean. Through statistical analyses, including a Wilcoxon signed rank test, this study finds significant differences in test achievements for tense, time prepositions (for and since), subjunctive moods, dative alternations, quantitative adjectives (many and much), gerunds, and third-person singular -s in the simple present tense. Based on results obtained, this study proposes which specific grammar features need intensive pedagogical intervention for production skills, including sentence writing and speaking. Several practical suggestions are provided for EFL instructors and researchers to make more effective use of speaking and writing activities as well as formative testing.