The direct active imperative uses a special method. First make the second-person active imperfect jussive, then remove its imperfect prefix tāʾ. If the next consonant already bears a vowel, no new initial hamzah is needed.
تُعَلِّمُ ← تُعَلِّمْ ← عَلِّمْ — teach! (Form II).
تَدْرُسُ ← تَدْرُسْ ← دْرُسْ — this leaves an initial consonant without a vowel, so deleting tāʾ alone is not enough.
The vowel state of the consonant now at the start decides whether hamzat al-waṣl is required.
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Why the direct active imperative is different
The Imperative: Active and Passive Paradigms and Formation Rules
The direct active imperative uses a special method. First make the second-person active imperfect jussive, then remove its imperfect prefix tāʾ. If the next consonant already bears a vowel, no new initial hamzah is needed.
تُعَلِّمُ ← تُعَلِّمْ ← عَلِّمْ — teach! (Form II).
تَدْرُسُ ← تَدْرُسْ ← دْرُسْ — this leaves an initial consonant without a vowel, so deleting tāʾ alone is not enough.
The vowel state of the consonant now at the start decides whether hamzat al-waṣl is required.