Language is a fundamental tool for learning science. This study highlights the importance of teacher knowledge in utilising language as a tool for knowledge generation in the classrooms. This case study examines elementary teachers' development of declarative, procedural, and epistemic knowledge related to using language, particularly focusing on how a three-year professional development programme centred around the Science Writing Heuristic (SWH) approach influences the development of these knowledge bases. Our findings indicate that the SWH PD positively influenced the teachers' knowledge development while there were disparities in knowledge development across individual teachers and knowledge aspects. We further discuss the changes in the teachers' knowledge and provide implications for future professional development and research regarding language use in science classrooms.