In يَقُولَانِ، يَقُولُونَ، تَقُولِينَ, the final nūn is the inflectional nūn, dropped in the subjunctive or jussive:
لَنْ يَقُولَا; لَمْ يَقُولُوا; لَنْ تَقُولِي.
By contrast, the nūn in يَقُلْنَ, تَقُلْنَ belongs to the feminine-plural ending, not to indicative inflection.
Thus لَمْ يَقُلْنَ retains its nūn. Confusing these two categories produces invalid forms.
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Inflectional Nūn Versus Feminine-Plural Nūn
Complete Conjugation of the Hollow Wāw Verb: Qāla–Yaqūlu
In يَقُولَانِ، يَقُولُونَ، تَقُولِينَ, the final nūn is the inflectional nūn, dropped in the subjunctive or jussive:
لَنْ يَقُولَا; لَمْ يَقُولُوا; لَنْ تَقُولِي.
By contrast, the nūn in يَقُلْنَ, تَقُلْنَ belongs to the feminine-plural ending, not to indicative inflection.
Thus لَمْ يَقُلْنَ retains its nūn. Confusing these two categories produces invalid forms.