The explanatory underlying form for قَالَ is قَوَلَ. The wāw is movable and follows fatḥah, so it changes to alif:
قَوَلَ → قَالَ.
The alif remains in forms such as قَالَا، قَالُوا، قَالَتْ. But suffixes such as -tu, -ta, -ti, -na produce forms including قُلْتُ، قُلْتَ، قُلْنَ.
The difference follows the syllable structure of the perfect forms: a weak long vowel cannot remain unchanged before every consonantal ending.
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Deriving the Active Perfect Qāla
Complete Conjugation of the Hollow Wāw Verb: Qāla–Yaqūlu
The explanatory underlying form for قَالَ is قَوَلَ. The wāw is movable and follows fatḥah, so it changes to alif:
قَوَلَ → قَالَ.
The alif remains in forms such as قَالَا، قَالُوا، قَالَتْ. But suffixes such as -tu, -ta, -ti, -na produce forms including قُلْتُ، قُلْتَ، قُلْنَ.
The difference follows the syllable structure of the perfect forms: a weak long vowel cannot remain unchanged before every consonantal ending.