We use must to say that we feel sure something is ture. You've been traveling all day. You must be tired. (Travelling is tiring and you've been travelling all day, so you must be tired) Carol must get very bored in her job. She does the same thing every day. We use can't to say that we feel sure something is nt possible. You've just had lunch. You can't be hungry already. (People are not normall..