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What happens to يَفْعُلُوْنَ after لَمْ?

Conjugating Past Absolute Negation with "Lam" (Active & Passive)

• Noon-e-I'raabi drops and the form becomes لَمْ يَفْعُلُوْا
• The ending becomes Fathah
• The middle root letter takes Kasrah
• Nothing changes