The feminine-plural imperfect forms يَفْعَلْنَ and تَفْعَلْنَ end with the nūn of feminine plural (nūn al-niswah), a pronominal suffix identifying the group.
By contrast, the nūn at the end of تَفْعَلِينَ is inflectional nūn retained in the indicative.
تَفْعَلِينَ — you (one woman) do; yāʾ plus inflectional nūn.
تَفْعَلْنَ — you (women) do; nūn al-niswah.
Before nūn al-niswah the last root consonant is sākin and the verb is indeclinable in this pattern. That nūn is not the indicative nūn that disappears under certain subjunctive/jussive conditions.
Opening interactive controls...
Feminine-plural nūn versus inflectional nūn
Complete Imperfect Paradigm: Active, Passive, Affirmative and Negative
The feminine-plural imperfect forms يَفْعَلْنَ and تَفْعَلْنَ end with the nūn of feminine plural (nūn al-niswah), a pronominal suffix identifying the group.
By contrast, the nūn at the end of تَفْعَلِينَ is inflectional nūn retained in the indicative.
تَفْعَلِينَ — you (one woman) do; yāʾ plus inflectional nūn.
تَفْعَلْنَ — you (women) do; nūn al-niswah.
Before nūn al-niswah the last root consonant is sākin and the verb is indeclinable in this pattern. That nūn is not the indicative nūn that disappears under certain subjunctive/jussive conditions.