You must be tired now。（你现在一定很累） 
You must have seen the film。（你一定看过这部电影）指的是你过去一定看过。 
考试常考这么几种： 
must have done （过去）一定做过某事 
It must have rained last night。（昨晚一定下雨了） 
cann't have done （过去）一定没有做过某事 
He cann't have seen the film，because he doesn's know anying about it。（他一定没看过那部电影，因为他对它一无所知） 
注意：千万不能用mustn't ，因为它是“禁止，绝不可以”的意思 
could have done （过去）本来可以做成某事实际上却没有 
I could have succeeded in the game if I hadn't have a cold。（如果我没感冒的话，我可能已经在比赛中成功了） 
should have done （过去）应该做某事而没有做 
You should have cleaned your room yesterday。（你昨天就应该把屋子打扫了）而实际上却没有