you could have done better, but you didn’t try your best.
he cannot have been to that town.
can he have got the book?
④might(may)+ have done：对过去发生的行为不太肯定的推测
he may not have finished the work.
if we had taken the other road, we might have arrived earlier.
⑤must+have done：对过去发生的行为肯定的推测。其否定式为：cannot have done
you must have seen the film.
you cannot have seen the film.
you needn’t have watered the flowers,for it is going to rain.
注意：didn’t need to(have to)do :没有必要做而实际上也没有做
i didn’t need to clean the windows.my sister did it 2 hours ago.
⑦should(ought to)+ have done：本来应该做而实际上又没有做。
you should have started earlier, but you didn’t.
she shouldn’t have taken away my measuring tape, for i wanted to use it.
he should have finished the work by now.
have to表示 “必须, 不得不”, 在这个意义上与must很接近, 但must表示的是说话人的主观看法, 而
have to表示的却是客观需要. have to比must有更多的形式。
the tv set is broken. i have to buy a new one.
the students will have to know how to use the computers.
注意：在回答must的问句时, 否定式常用need not(needn’t)或don’t have to表示 “不必”。而不用
must not, 因为must not表示 “不可以”。
—did you use to go there to see your brother? /used you to go there to see your brother?
—yes, i did(used to). /no , i didn’t (usedn’t).
we had better go now.
其否定式为had better not
类别 用法 例句
if引导的条件从句 与现在事实相反 从句动词：过去式（be用were）
主句动词：should/would/could/might+动词原形 if he were here, he would help us.
主句动词：should/would/could/might+have+过去分词 if i had been free, i would have visited you.
与将来事实相反 从句动词：过去式 / should+动词原形 / were+不定式