This study investigates subject-verb agreement errors in academic writing by BS English students, aiming to enhance their writing proficiency. Despite learning grammatical rules like subject-verb agreement, students frequently struggle to apply these rules correctly. Using a mixed-method approach, data was collected both quantitatively and qualitatively. The findings revealed four types of errors: subject-verb agreement of person (SV1), number (SV2), subject (SV3), and coordinated subject (SV4). No errors were observed for notional agreement and proximity (SV5). Errors in SV1, representing a person's simple subject-verb agreement, were the most frequent. The dominant source of these errors was identified as intralingual factors. The study concludes by offering practical solutions to help students and educators address these challenges effectively, contributing to improved academic writing skills for BS English learners.