Three groups of EFL freshman students were concurrently enrolled in a grammar and a writing course. One group was taught the grammar and writing courses by the same instructor; the other two groups were taught grammar and writing by two different instructors using the same textbook but different instructional and assessment techniques. The study aims to find out which group made higher gains in grammatical knowledge and writing skill development. It also aimed to find out the effects of instructor qualifications, assessment, and pedagogical variables on students' grammatical competence and writing skill development. Comparisons of the grammar and writing post-tests showed significant differences between the three groups in the writing and grammar post-test mean scores. There were strong correlations between the grammar and writing post-test scores. Performance of the Group that received a combination of writing and grammar instruction by the same instructor (Instructor A) was the highest. The relationship between grammar and writing instruction seems to be reciprocal: writing instruction affects grammatical competence and grammatical knowledge affects writing skill development. Better achievements were made when both courses were taught by the same instructor as she can make the right connections between what is taught in both courses, which specific structures and skills should be emphasized. The instructors' qualifications, pedagogical system, educational and professional experience, the integration of online instruction, the type of error correction and instant feedback given to the students and the formative assessment technique used were significantly more effective than writing/grammar instruction that depended on the textbook alone. These variables proved to be important for enhancing the grammatical knowledge and writing quality of unskilled, low ability EFL students and resulted in a significant improvement in their grammar and writing post-test scores.