لَا يَكْتُبُ — لَنْ يَكْتُبَ — لَمْ يَكْتُبْ
Negative lā here leaves indicative, lan governs subjunctive with future negation, and lam governs jussive with past-time negation.
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Lan, Lā and Lam Compared
The Subjunctive Imperfect and Its Governors
لَا يَكْتُبُ — لَنْ يَكْتُبَ — لَمْ يَكْتُبْ
Negative lā here leaves indicative, lan governs subjunctive with future negation, and lam governs jussive with past-time negation.