This study aims to examine how Chinese learners develop in their use and misuse of English modal verbs from Grade 7 to 9. Specifically, it examines form-function connections and explores the factors behind learners' development. The main focus is on the modal verbs "can," "could," "will," "would," "must," and "should." The learner corpus consists of essay samples selected from the writings of 600 participants, stratified across three levels (Grade 7, 8, and 9). The developmental patterns of modal verbs are assessed in terms of completeness, robustness, and proximity to the target language. The analysis of misuse looks at semantic and pragmatic errors. The findings indicate that there are considerable variations in the form-function connections of the six modal verbs at the initial and subsequent stages. The influencing factors include L1 transfer, the influence of textbooks, L2 complexity, universal learning mechanisms, and teachers' instructions.