يَقُولُ — لَنْ يَقُولَ — لَمْ يَقُلْ
Sound-final concerns the final radical, not whether the entire root lacks weak letters. Yaqūlu has an internal weak vowel but final lām is sound. In lam yaqul, internal shortening is morphological, while final sukūn marks jazm.
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What Sound-Final Means
Four Patterns of Imperfect Mood Marking
يَقُولُ — لَنْ يَقُولَ — لَمْ يَقُلْ
Sound-final concerns the final radical, not whether the entire root lacks weak letters. Yaqūlu has an internal weak vowel but final lām is sound. In lam yaqul, internal shortening is morphological, while final sukūn marks jazm.