The source gives this construction from the active Mudhaari' form:
1. Remove the Mudhaari' prefix.
2. Add open Hamzah at the beginning.
3. Give the middle root letter Fathah.
4. Give the final root letter one Dhammah.
5. The result is أَفْعَلُ.
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How the Masculine Form Is Built
Derived Nouns: Comparative/Superlative Noun (Ism Tafdheel - Masculine)
The source gives this construction from the active Mudhaari' form:
1. Remove the Mudhaari' prefix.
2. Add open Hamzah at the beginning.
3. Give the middle root letter Fathah.
4. Give the final root letter one Dhammah.
5. The result is أَفْعَلُ.