Arabic does not begin a spoken word with a vowelless consonant. If removing the second-person prefix leaves such a consonant, a hamzat al-waṣl is supplied to make initial pronunciation possible.
تَضْرِبُ ← تَضْرِبْ ← ضْرِبْ ← اِضْرِبْ — strike!
تَنْصُرُ ← تَنْصُرْ ← نْصُرْ ← اُنْصُرْ — help!
This initial hamzah is not an original root consonant. It makes the beginning pronounceable, and its kasrah or ḍamma is determined by the morphological rule.
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Why hamzat al-waṣl is needed
The Imperative: Active and Passive Paradigms and Formation Rules
Arabic does not begin a spoken word with a vowelless consonant. If removing the second-person prefix leaves such a consonant, a hamzat al-waṣl is supplied to make initial pronunciation possible.
تَضْرِبُ ← تَضْرِبْ ← ضْرِبْ ← اِضْرِبْ — strike!
تَنْصُرُ ← تَنْصُرْ ← نْصُرْ ← اُنْصُرْ — help!
This initial hamzah is not an original root consonant. It makes the beginning pronounceable, and its kasrah or ḍamma is determined by the morphological rule.