Ism mamdud is a noun ending in an extra alif followed by hamzah, اء, such as رَفَّاءٌ and حَمْرَاءُ.
Ism maqsur is a noun ending in alif maqsurah, ى, such as فَتًى and مُصْطَفَى.
Do not apply one common mechanical change to both kinds. First identify the ending; then use the relevant dual or plural rule.
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Dual and Plural of Mamdud and Maqsur Nouns
Ism mamdud is a noun ending in an extra alif followed by hamzah, اء, such as رَفَّاءٌ and حَمْرَاءُ.
Ism maqsur is a noun ending in alif maqsurah, ى, such as فَتًى and مُصْطَفَى.
Do not apply one common mechanical change to both kinds. First identify the ending; then use the relevant dual or plural rule.