One of the most difficult areas to address in required college English classes for students and teachers alike is composition accuracy at the sentential level, that is, sentence grammar. The purpose of this study is to explore the effect of explicit grammar instruction on Korean students' perceptions of their writing and editing skills at the sentential level in a freshman reading and writing course. The study participants comprised of 15 students enrolled in a required intermediate-level freshman English reading and writing course during the winter vacation semester at a top-ranking university in Seoul. The students completed a pre-intervention writing assignment prior to receiving sentence grammar instruction, and a similar but slightly different post-intervention assignment after receiving the instruction. The intervention was provided in the form of a set of workbooks which the students read and studied for homework over a period of a week. Aside from answering students' questions on the workbooks, no other sentence grammar instruction was provided. Upon submitting the post-intervention writing assignment, the students completed an online survey anonymously to reflect on their experience of the overall task. The results of the survey point to a positive impact of the intervention on the participants' perceptions of their writing and editing abilities. This study is significant for raising the issue that explicit grammar instruction delivered in the form of workbooks could have a beneficial role in foreign language writing pedagogy.