The source for لَمْ يَقُلْ is يَقُولُ. Jussive sukūn creates a meeting with the already long/vowelless middle wāw; the weak long vowel is elided.
لَمْ يَقُلْ — he did not say.
لَمْ يُقَلْ — it was not said.
In the passive the alif in يُقَالُ likewise disappears under jussive formation.
The cause in both cases is meeting of two sukūns. Elision does not erase the active/passive distinction, since the remaining vowels still differ.
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Why the Jussive Shortens Both Voices
Complete Conjugation of the Hollow Wāw Verb: Qāla–Yaqūlu
The source for لَمْ يَقُلْ is يَقُولُ. Jussive sukūn creates a meeting with the already long/vowelless middle wāw; the weak long vowel is elided.
لَمْ يَقُلْ — he did not say.
لَمْ يُقَلْ — it was not said.
In the passive the alif in يُقَالُ likewise disappears under jussive formation.
The cause in both cases is meeting of two sukūns. Elision does not erase the active/passive distinction, since the remaining vowels still differ.