لَمْ makes the imperfect jussive, whereas لَنْ makes it subjunctive.
RootJussiveSubjunctive
ق و للَمْ يَقُلْلَنْ يَقُولَ
ب ي علَمْ يَبِعْلَنْ يَبِيعَ
خ و فلَمْ يَخَفْلَنْ يَخَافَ
Writing *lan yaqul* wrongly carries the deleted-vowel jussive form into a subjunctive construction. Keep both the governing particle and the appropriate weak-letter change in view.
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Advanced Sarf Application: Ambiguous Forms, Weak-Letter Reasoning and Error Correction
لَمْ makes the imperfect jussive, whereas لَنْ makes it subjunctive.
RootJussiveSubjunctive
ق و للَمْ يَقُلْلَنْ يَقُولَ
ب ي علَمْ يَبِعْلَنْ يَبِيعَ
خ و فلَمْ يَخَفْلَنْ يَخَافَ
Writing *lan yaqul* wrongly carries the deleted-vowel jussive form into a subjunctive construction. Keep both the governing particle and the appropriate weak-letter change in view.