The source rule starts from the third-person masculine singular imperfect. Identify the root radicals and the initial imperfect prefix. Then:
1. Remove the imperfect prefix.
2. Replace it with مَ (mīm with fatḥah).
3. If the second radical had ḍammah, change it to fatḥah; otherwise keep its vowel.
4. Add the appropriate nominal tanwīn at the end of the displayed citation form.
Thus يَنْصُرُ produces مَنْصَرٌ, while يَضْرِبُ produces مَضْرِبٌ.
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Derivation from the imperfect
Ism al-Ẓarf: Meaning, Patterns, Paradigm and Derivation
The source rule starts from the third-person masculine singular imperfect. Identify the root radicals and the initial imperfect prefix. Then:
1. Remove the imperfect prefix.
2. Replace it with مَ (mīm with fatḥah).
3. If the second radical had ḍammah, change it to fatḥah; otherwise keep its vowel.
4. Add the appropriate nominal tanwīn at the end of the displayed citation form.
Thus يَنْصُرُ produces مَنْصَرٌ, while يَضْرِبُ produces مَضْرِبٌ.