In يَقُولُ, qāf carries ḍammah and the following wāw is still present; يَبِيعُ likewise contains yāʾ after kasrah-bearing bāʾ. The weak middle radical survives as a long vowel.
In قُلْتُ and بِعْتُ, the medial weak letter is absent because the inflection creates a different quiescent-letter environment.
A derivation should distinguish whether the weak radical is still present from whether its original short vowel has changed location.
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Transfer Is Not Elision
Hollow Verbs: Seven Iʿlāl Rules and Three Short Paradigms
In يَقُولُ, qāf carries ḍammah and the following wāw is still present; يَبِيعُ likewise contains yāʾ after kasrah-bearing bāʾ. The weak middle radical survives as a long vowel.
In قُلْتُ and بِعْتُ, the medial weak letter is absent because the inflection creates a different quiescent-letter environment.
A derivation should distinguish whether the weak radical is still present from whether its original short vowel has changed location.