English writing proficiency is a crucial skill in academic and professional contexts, yet learners from non-English-speaking backgrounds often face significant challenges in mastering it. For Tamil learners in India, the influence of the mother tongue (L1) plays a vital role in shaping English (L2) writing skills. This study investigates the impact of Mother Tongue Instruction (MTI) as a pedagogical strategy to enhance English writing competence among secondary-level Tamil learners. Employing a mixed-methods approach, 60 students from two government-aided schools in Tamil Nadu were divided into an experimental group (receiving MTI-integrated English writing lessons) and a control group (receiving English-only instruction). Data were collected through pre- and post-tests, error analysis, and semi-structured interviews with both learners and teachers. Statistical analysis using paired-sample t-tests revealed that the MTI group demonstrated a significantly greater improvement in writing organization, grammatical accuracy, and vocabulary richness (p < 0.05). Qualitative findings indicated that learners perceived MTI as reducing anxiety, clarifying conceptual understanding, and enabling effective idea generation. The study concludes that strategic use of the mother tongue can facilitate cognitive transfer, scaffold complex writing tasks, and promote bilingual competence without impeding English fluency. The paper recommends integrating MTI in bilingual education policies, particularly for regions with strong linguistic identities such as Tamil Nadu.

Keywords:
Mother Tongue Instruction (MTI), English Writing Skills, Tamil Learners, Bilingual Education, Second Language Acquisition, Error Analysis, Language Pedagogy