Bab 4 and Bab 5: The Key Difference

Derived Trilateral (with Hamzah) - Bab 5: If'eelaal (Idheemam)

Compare the two adjacent doors in their active past forms.

Visible differenceExampleBab
No internal Alif before the doubled final letterاِحْمَرَّBab 4 — If'ilaal
Added internal Alif before the doubled final letterاِدْهَامَّBab 5 — If'eelaal

The internal Alif in اِدْهَامَّ is the clearest visual difference taught here.