Arabic verbs are classified by the number of root letters and whether any extra letters have been added. A فعل ثلاثي مجرد (trilateral simple/unaugmented verb) has exactly three root letters with no additions. A فعل مزيد (augmented verb) is one where extra letters have been added to the root, and a فعل رباعي (quadrilateral verb) has four root letters.
The verb تدرُسُ (you study / she studies) is the present tense (المضارع) form of درس. The root letters are د - ر - س, which are exactly three letters. The تـ prefix is simply the present-tense conjugation marker, not an augmentation of the root. Since the root has three letters and no extra letters have been added to the base form درس, this verb is ثلاثي مجرد (trilateral unaugmented). It is not جامد (frozen/non-conjugating) because it conjugates freely across all tenses.
Exam tip: To determine a verb's type, always strip away conjugation prefixes and suffixes to find the root. Count the root letters: three letters with no additions means ثلاثي مجرد.
