Incorrect: “Because بِالْمَسَاجِدِ has kasrah, it must also admit tanwīn.”
Correct: nouns with al- do not take tanwīn. Nor does the muḍāf in مَسَاجِدِ الْمَدِينَةِ.
The return of genitive kasrah and the presence of tanwīn are separate matters.
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Correcting a tanwīn error
Restoring Declension: Indefinite Names, Al- and Iḍāfah
Incorrect: “Because بِالْمَسَاجِدِ has kasrah, it must also admit tanwīn.”
Correct: nouns with al- do not take tanwīn. Nor does the muḍāf in مَسَاجِدِ الْمَدِينَةِ.
The return of genitive kasrah and the presence of tanwīn are separate matters.