Lam Makes the Mudhaari' Jussive

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

Lam is a jussive particle.
Across the 14-form Mudhaari' grid, its structural effect falls into three groups:

What happensForms
final Dhammah becomes Sukoon5
Noon-e-I'raabi drops7
form remains unchanged2

Check: 5 + 7 + 2 = 14.