Complex sentence: Mrs Beech's attitude to William is revealed as disdainful and waspish, and when she hears that the belt has not been brought home, she smacks him, thinking that he deliberately left it behind, and sends him to his room.
Split into simpler sentences: Mrs Beech's attitude to William is revealed as disdainful and waspish. When she hears that the belt has not been brought home, she smacks him, thinking that he deliberately left it behind, and sends him to his room.