Five Mudhaari' forms normally end with Dhammah. With لَنْ, that final Dhammah changes to Fathah.
After LanBefore Lan
لَنْ يَفْعُلَيَفْعُلُ
لَنْ تَفْعُلَتَفْعُلُ — feminine Ghaa'ib singular
لَنْ تَفْعُلَتَفْعُلُ — masculine Haadhir singular
لَنْ أَفْعُلَأَفْعُلُ
لَنْ نَفْعُلَنَفْعُلُ
Rule: Dhammah → Fathah in these 5 positions.
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Rule of Emphatic Negation in Future with "Lan" (لَنْ)
Five Mudhaari' forms normally end with Dhammah. With لَنْ, that final Dhammah changes to Fathah.
After LanBefore Lan
لَنْ يَفْعُلَيَفْعُلُ
لَنْ تَفْعُلَتَفْعُلُ — feminine Ghaa'ib singular
لَنْ تَفْعُلَتَفْعُلُ — masculine Haadhir singular
لَنْ أَفْعُلَأَفْعُلُ
لَنْ نَفْعُلَنَفْعُلُ
Rule: Dhammah → Fathah in these 5 positions.