The challenge for teacher professional development (TPD) to achieve the educational reform agenda demands that teacher educators play an innovative role in connecting teachers’ theory-practice gaps. This study investigates the personal practical theories (PPTs) of the English-language (EL) student-teacher about grammar instruction during their initial experiences in the university’s school-based practice course and explores how they apply the Communicative Language Teaching (CLT) - a prescribed approach to teaching EL, imposed by Thailand’s national curriculum. Phrase one utilized the self-survey questionnaire to retrieve PPTs about grammar instruction from twenty-six EL student-teacher from Yala Rajabhat University during their fieldwork practicum in Thailand’s southernmost schools. In phase two, the four critical samples were observed in their twelve classrooms regarding grammar roles in EL instructional practices. Findings show that before the practicum, the participants held the CLT-oriented PPTs, showing a certain commitment to applying CLT theory into practice. However, the aspects of formal instruction of the grammar and grammar-focused error correction observed in their teaching practices were not compatible with their professed PPTs. The Thai EL studentteacher practices were driven by a core perception that grammar is the foundation for communication unsupportive to CLT. This study recommends that teacher education should employ the innovative strategy of reflective practice in promoting the student-teacher PPTs and practices early in the practicum course. To enhance teachers’ learning to teach innovatively and warrant the success of instructional reform, the insightful understanding of teachers’ cognition should be taken as a crucial component of teacher education.