The original form taught here is قَلْسَيَ.
Its final Ya is vowelled and the letter before it has Fathah.
Because of that, the Ya changes to Alif:
قَلْسَيَ → قَلْسَىٰ.
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Why قَلْسَيَ Becomes قَلْسَىٰ
Attached Trilateral (Mujarrad) - Bab 7: Fa'laah & Weak Root Rules
The original form taught here is قَلْسَيَ.
Its final Ya is vowelled and the letter before it has Fathah.
Because of that, the Ya changes to Alif:
قَلْسَيَ → قَلْسَىٰ.