If an ism maqsur has more than three letters, change the final alif maqsurah to ya before adding the dual ending.
مُصْطَفَى → مُصْطَفَيَانِ
حُبْلَى → حُبْلَيَانِ
The learner-friendly procedure is: identify a maqsur noun longer than three letters, replace final ى with ي, and then add انِ.
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Dual and Plural of Mamdud and Maqsur Nouns
If an ism maqsur has more than three letters, change the final alif maqsurah to ya before adding the dual ending.
مُصْطَفَى → مُصْطَفَيَانِ
حُبْلَى → حُبْلَيَانِ
The learner-friendly procedure is: identify a maqsur noun longer than three letters, replace final ى with ي, and then add انِ.