لَمْ negates and makes the imperfect jussive, as does prohibitive لا. Weak-final verbs can therefore lose their final weak letter in prohibition.
Word / formMeaning / explanation
لَمْ يَرْمِhe did not throw.
لا تَرْمِdo not throw.
In contrast, لَنْ يَرْمِيَhe will not throw, is subjunctive, not jussive.
Although lam and lan both negate, their time reference and grammatical effect on the weak final are different.
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Defective Verbs: Six Iʿlāl Rules and Four Short Paradigms
لَمْ negates and makes the imperfect jussive, as does prohibitive لا. Weak-final verbs can therefore lose their final weak letter in prohibition.
Word / formMeaning / explanation
لَمْ يَرْمِhe did not throw.
لا تَرْمِdo not throw.
In contrast, لَنْ يَرْمِيَhe will not throw, is subjunctive, not jussive.
Although lam and lan both negate, their time reference and grammatical effect on the weak final are different.