Muʿallimātun loses tanwīn when it becomes the first term of an iḍāfa: muʿallimātu al-madrasati.
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Loss of Tanwīn with Iḍāfa
Tanwīn: Five Types, Their Function, and Omission
Muʿallimātun loses tanwīn when it becomes the first term of an iḍāfa: muʿallimātu al-madrasati.