Three-letter maqṣūr from wāw restores wāw; three-letter from yāʾ uses yāʾ; longer maqṣūr generally uses yāʾ.
The written final alif alone cannot establish its origin.
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Three Decisions for Maqṣūr
The Dual Noun: Inflection, Maqṣūr, Mamdūd and Iḍāfah
Three-letter maqṣūr from wāw restores wāw; three-letter from yāʾ uses yāʾ; longer maqṣūr generally uses yāʾ.
The written final alif alone cannot establish its origin.