The nūn al-niswah in feminine plural imperfect forms is not the same as the indicative inflectional nūn. It is preserved in an imperative because it identifies a feminine-plural subject.
تَفْعَلْنَ → اِفْعَلْنَ — you women do.
يَفْعَلْنَ → لِيَفْعَلْنَ — let those women do.
But in تَفْعَلِينَ → اِفْعَلِي, the inflectional nūn is deleted. A written nūn can therefore perform two different morphological functions, with different rules for imperative formation.
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The feminine-plural nūn is not an inflectional nūn
The Imperative: Active and Passive Paradigms and Formation Rules
The nūn al-niswah in feminine plural imperfect forms is not the same as the indicative inflectional nūn. It is preserved in an imperative because it identifies a feminine-plural subject.
تَفْعَلْنَ → اِفْعَلْنَ — you women do.
يَفْعَلْنَ → لِيَفْعَلْنَ — let those women do.
But in تَفْعَلِينَ → اِفْعَلِي, the inflectional nūn is deleted. A written nūn can therefore perform two different morphological functions, with different rules for imperative formation.