1)I don’t know why she isn’t here. She must / might have forgotten, or maybe she’s ill. * 2)They look unhappy. They must / can’t have won the race. *
3)My bike is gone. Someone can’t / could have stolen it last night. *
4)He went to the doctor’s. He must / can’t have been ill. *
5)I could / must have seen her. I’m not sure. I don’t really remember what she looks like! *