Complex sentence: Then DuanMu accidentally knocks YunHai's head on the staircase rail that causes YunHai to remember everything about YuXun: his promises with her, their fight, their bickering, their sacrifices for one another, and, most importantly, their extreme love for each other.
Split into simpler sentences: Then DuanMu accidentally knocks YunHai's head on the staircase rail that causes YunHai to remember everything about YuXun. His promises with her, their fight, their bickering, their sacrifices for one another and most importantly their extreme love for each other.