هُمَا يَكْتُبَانِ — لَنْ يَكْتُبَا — لَمْ يَكْتُبَا
The third-person dual retains nūn in the indicative and drops it in both subjunctive and jussive. The dual alif remains.
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Dual Third Person: Three Moods
Four Patterns of Imperfect Mood Marking
هُمَا يَكْتُبَانِ — لَنْ يَكْتُبَا — لَمْ يَكْتُبَا
The third-person dual retains nūn in the indicative and drops it in both subjunctive and jussive. The dual alif remains.