The development of grammatical competence is an important component of professional training for future Arabic language teachers. However, the successful completion of grammar exercises or the memorization of grammatical rules does not always indicate the learner’s ability to apply these rules accurately in oral and written communication. Therefore, an effective diagnostic assessment should identify not only what grammatical rules students know, but also how they understand, analyze, apply, and correct grammatical structures. This article examines the methodological foundations of diagnosing Arabic grammatical competence among future Arabic language teachers. Particular attention is given to the selection of diagnostic tasks, the identification of typical grammatical difficulties, and the criteria for determining different levels of grammatical competence. The proposed diagnostic framework includes tasks based on grammatical recognition, selection of correct forms, error identification and correction, sentence transformation, and independent sentence production. The article argues that a systematic diagnostic approach can provide more reliable information about students’ actual grammatical competence and can serve as a basis for organizing subsequent instructional activities.