Complex sentence: After suffering a head injury however, he has developed a rare form of amnesia that causes him to forget anything that happened to him the previous day, which makes it hard for him to know whom to trust, or if he even knows them at all.
Split into simpler sentences: After suffering a head injury however, he has developed a rare form of amnesia that causes him to forget anything that's happened to him the previous day. This makes it hard for him to know who to trust, or if he even knows them at all.