يَكْتُبْنَ — لَيَكْتُبَنَّ
Nūn al-niswah is a subject pronoun and builds the verb on sukūn; directly attached emphatic nūn is not subject and builds it on fatḥ.
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Contrast the Two Nūns
The Imperfect Verb: Recognition, Time, Prefixes and Mood
يَكْتُبْنَ — لَيَكْتُبَنَّ
Nūn al-niswah is a subject pronoun and builds the verb on sukūn; directly attached emphatic nūn is not subject and builds it on fatḥ.