The source gives this construction rule from the Mudhaari' form:
1. Remove the Mudhaari' prefix.
2. Add an open Meem at the beginning.
3. If the middle root letter in Mudhaari' has Dhammah or Fathah, use مَفْعَلٌ.
4. If it has Kasrah, keep that vowel and use مَفْعِلٌ.
5. Give the final root letter Tanween.
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How Ism Dharf Is Formed
Derived Nouns: Noun of Place and Time (Ism Dharf)
The source gives this construction rule from the Mudhaari' form:
1. Remove the Mudhaari' prefix.
2. Add an open Meem at the beginning.
3. If the middle root letter in Mudhaari' has Dhammah or Fathah, use مَفْعَلٌ.
4. If it has Kasrah, keep that vowel and use مَفْعِلٌ.
5. Give the final root letter Tanween.