This project aims to expand on existing research on the use of AI tools in writing education by not only examining AI-driven pedagogical interventions and their effects but also by focusing on the learning process and potential longitudinal changes in the development of writing skills in English as a foreign language. The research intends to show how individual differences (such as cognitive differences and differences in motivation) influence the development of writing skills and the use of generative AI, and what the effects are of using generative AI on various text characteristics.