يَكْتُبُ — لَنْ يَكْتُبَ — لَمْ يَكْتُبْ
The imperfect normally inflects for mood because of its classical grammatical resemblance to the noun.
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Why the Imperfect Is Normally Muʿrab
The Imperfect Verb: Recognition, Time, Prefixes and Mood
يَكْتُبُ — لَنْ يَكْتُبَ — لَمْ يَكْتُبْ
The imperfect normally inflects for mood because of its classical grammatical resemblance to the noun.