Read each example as one complete “not yet” statement.
● لَمَّا يَكْتُبْ زَيْدٌ — Zayd has not written yet. يَكْتُبْ ends in sukun.
● لَمَّا تَطْلُعِ الشَّمْسُ — The sun has not risen yet. The connecting kasrah is explained next.
● لَمَّا أَقْرَأْ هَذَا الْكِتَابَ — I have not read this book yet. أَقْرَأْ ends in sukun.
The doer can change, but لَمَّا keeps the same time meaning.