Used to. Don't have to and mustn't. Have to, don't have to and mustn't. Can and could 2. Can and could 1. Complete the sentences with must or mustn't. Complete the school rules with must or mustn't. Shoud, must, don't have to, mustn't. Modals - Exercise. Modals - should / shouldnґt / mustnґt. SCHOOL RULES (can, must/mustn't) Rules with must/mustn't - interactive book.