لَمَّا uses the same three Mudari Akhaff ending rules learned in the previous level.
● First group: يَصِلُ ← لَمَّا يَصِلْ — final dammah becomes sukun.
● Second group: يَكْتُبُونَ ← لَمَّا يَكْتُبُوا — the added inflectional nun is removed.
● Third group: تَكْتُبْنَ ← لَمَّا تَكْتُبْنَ — no visible change; the feminine-plural nun remains.
The meaning in all three is “not yet,” but the visible ending follows its own group.