The third taught situation is a noun ending in additional alif-nun that has no feminine counterpart. رَحْمٰنُ is the established example. It should be recognized respectfully as the taught lexical example; do not invent a feminine form for it.
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Situation 3: A Noun with No Feminine Counterpart
Cause of Ghayr Munsarif: Additional Alif and Nun
The third taught situation is a noun ending in additional alif-nun that has no feminine counterpart. رَحْمٰنُ is the established example. It should be recognized respectfully as the taught lexical example; do not invent a feminine form for it.