Complex sentence: After the holidays, Harry begins working with Professor Lupin to fight Dementors with the Patronus Charm; he is moderately successful, but still not entirely confident in his ability to ward them off.
Split into simpler sentences: After the holidays, Harry begins working with Professor Lupin to fight Dementors with the Patronus Charm. He is moderately successful but still not entirely confident in his ability to ward them off.