Complex sentence: If it has a sukun, then it is heavy if the first preceding letter without a sukun has a fatḥah or ḍammah and light when the first preceding letter without a sukun has a kasrah.
Split into simpler sentences: If it has a sukun, then it is heavy if the first preceding letter without a sukun has a fatḥah or ḍammah. It is light when the first preceding letter without a sukun has a kasrah.