The final nūn in يَفْعَلْنَ and تَفْعَلْنَ is the nūn of the feminine plural (nūn al-niswah), not an inflectional nūn. As a marker of feminine-plural person, it is not deleted by lan.
يَفْعَلْنَ ← لَنْ يَفْعَلْنَ — the women will not do.
تَفْعَلْنَ ← لَنْ تَفْعَلْنَ — you women will not do.
Contrast يَفْعَلُونَ ← لَنْ يَفْعَلُوا, where the inflectional nūn does drop. Not every final nūn has the same function.
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Why the feminine-plural nūn remains
Negating the Imperfect with Lan and Lam: Subjunctive, Jussive and Full Paradigms
The final nūn in يَفْعَلْنَ and تَفْعَلْنَ is the nūn of the feminine plural (nūn al-niswah), not an inflectional nūn. As a marker of feminine-plural person, it is not deleted by lan.
يَفْعَلْنَ ← لَنْ يَفْعَلْنَ — the women will not do.
تَفْعَلْنَ ← لَنْ تَفْعَلْنَ — you women will not do.
Contrast يَفْعَلُونَ ← لَنْ يَفْعَلُوا, where the inflectional nūn does drop. Not every final nūn has the same function.