Complex sentence: On the way to Hogwarts, a Dementor boards the train, causing Harry to faint, but he is helped by the new Defence Against the Dark Arts teacher, Remus Lupin.
Split into simpler sentences: On the way to Hogwarts a Dementor boards the train, causing Harry to faint. Following the incident, Harry is helped by the new Defence Against the Dark Arts teacher Remus Lupin.