In يَبِيعَانِ، يَبِيعُونَ، تَبِيعِينَ, final nūn is the inflectional nūn. It drops after لَنْ and لَمْ: لَنْ يَبِيعَا, لَمْ يَبِيعُوا, لَنْ تَبِيعِي.
But يَبِعْنَ, تَبِعْنَ end in the feminine-plural nūn, which links the verb to women as its subjects.
That nūn remains in لَمْ يَبِعْنَ and لَنْ يَبِعْنَ. The two kinds of nūn cannot be treated as interchangeable.
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Inflectional Nūn and the Feminine-Plural Nūn
Complete Paradigms of the Hollow Yāʾ Verb: Bāʿa–Yabīʿu
In يَبِيعَانِ، يَبِيعُونَ، تَبِيعِينَ, final nūn is the inflectional nūn. It drops after لَنْ and لَمْ: لَنْ يَبِيعَا, لَمْ يَبِيعُوا, لَنْ تَبِيعِي.
But يَبِعْنَ, تَبِعْنَ end in the feminine-plural nūn, which links the verb to women as its subjects.
That nūn remains in لَمْ يَبِعْنَ and لَنْ يَبِعْنَ. The two kinds of nūn cannot be treated as interchangeable.