The active participle قَائِلٌ is explained from the pattern-form قَاوِلٌ: in the فَاعِل pattern, an original medial wāw after the long alif becomes hamzah.
The underlying passive participle is مَقْوُولٌ; vowel transfer and reduction of consecutive wāws yield the standard مَقُولٌ.
قَائِلٌ — one who says; مَقُولٌ — something said.
Each pattern follows its own rule; the hamzah of the active participle must not be inserted indiscriminately into the passive participle.
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Why Qāʾil but Maqūl?
Complete Conjugation of the Hollow Wāw Verb: Qāla–Yaqūlu
The active participle قَائِلٌ is explained from the pattern-form قَاوِلٌ: in the فَاعِل pattern, an original medial wāw after the long alif becomes hamzah.
The underlying passive participle is مَقْوُولٌ; vowel transfer and reduction of consecutive wāws yield the standard مَقُولٌ.
قَائِلٌ — one who says; مَقُولٌ — something said.
Each pattern follows its own rule; the hamzah of the active participle must not be inserted indiscriminately into the passive participle.