Distinguish inflectional nūn, emphatic nūn, and the feminine plural subject suffix.
Nūn typeFormFunction
Indicative nūnيَكْتُبُونَIndicative ending
Heavy emphaticلَيَكْتُبَنَّStrong emphasis
Light emphaticلَيَكْتُبَنْLight emphasis
Feminine pluralيَكْتُبْنَFeminine plural subject
Their grammar differs. The light emphatic nūn is not available in every paradigm cell; preserve unavailable slots rather than inventing forms.