Group 1: Five Forms Take Sukoon

Rule of Past Absolute Negation with "Lam" (لَمْ) & Weak Letters

Five forms normally end with Dhammah. After لَمْ, that final Dhammah becomes Sukoon.

After LamBefore Lam
لَمْ يَفْعُلْيَفْعُلُ
لَمْ تَفْعُلْتَفْعُلُ — feminine Ghaa'ib singular
لَمْ تَفْعُلْتَفْعُلُ — masculine Haadhir singular
لَمْ أَفْعُلْأَفْعُلُ
لَمْ نَفْعُلْنَفْعُلُ

Rule: Dhammah → Sukoon in these 5 positions.