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Why do يَفْعُلْنَ and تَفْعُلْنَ remain unchanged in this paradigm?

Rule of Emphatic Negation in Future with "Lan" (لَنْ)

• Their final Noon belongs to the feminine-plural subject pronoun and remains fixed here
• They are past-tense forms
• They have no vowel marks
• لَنْ never affects plural forms