When an ism maqsur consists of three letters, its final alif represents an original letter. In the dual, that original letter returns.
عَصًا → عَصَوَانِ
The returned original letter is waw.
فَتًى → فَتَيَانِ
The returned original letter is ya.
Therefore, do not turn every three-letter maqsur ending into ya automatically; the dual reveals whether waw or ya returns.
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Dual and Plural of Mamdud and Maqsur Nouns
When an ism maqsur consists of three letters, its final alif represents an original letter. In the dual, that original letter returns.
عَصًا → عَصَوَانِ
The returned original letter is waw.
فَتًى → فَتَيَانِ
The returned original letter is ya.
Therefore, do not turn every three-letter maqsur ending into ya automatically; the dual reveals whether waw or ya returns.