This thesis explores the development and validation of supplementary intervention materials aimed at enhancing the grammatical competence of Grade 7 students at Bacnotan National High School for the 2016-2017 academic year. The study found that students exhibited fairly competent grammar skills, but also identified a significant difference in pre-test and post-test scores, suggesting that the intervention materials were effective. Recommendations include providing more student-centered activities to improve communication skills, and advocating for the use of these supplementary materials across the province.