大學英語六級基礎語法知識：情態(tài)動詞
1、情態(tài)動詞的完成式
1)must have done——表示對過去的事情較有把握的、肯定性的推測，常譯為“一定，準是，肯定”，其否定形式是can’t/couldn’thave done，意為“決不可能，不太可能”。
e.g. Since the ditch is full of water, it must have rained last night.
2)should/ought to have done——表示過去應該做的卻沒有做，常含有后悔、責備、埋怨的口吻，意為“本應該”，“應該就好了”; e.g. With all the work finished, I should have gone to your birthday party last night.
3)shouldn’t/ought not to have done——表示過去不該做的事卻給做了，也常含有責備、不滿之意，意為“本不該”。
e.g. You shouldn’t have gone back to work yesterday without the doctor’s permission.
4)may/might have done——表示對過去的事情的推測，但不十分有把握，意為“可能，或許已經(jīng)”。 e.g. A traffic accident happened yesterday and a driver may have been hurt.
5)could have done——表示過去有可能或有能力做某事，而實際未做、未能實現(xiàn)的事;有時也表示輕度批評，意為“本來可以，完全可以”。 e.g. The gas leakage could have been avoided if the gas company took immediate measures after the warning call.
6)needn’t have done——表示過去做了某事，但沒有做的必要，意為“其實沒有必要”。 e.g. I needn’t have bought all that wine——only three people came.
2、可作情態(tài)動詞用的短語
would rather, would sooner, would (just) as soon, had rather, had better, had sooner, cannot but, may/might as well
[例1]You________ all those calculations! We have a computer to do that sort of thing.
A. needn’t have done B. must not have done
C. shouldn’t have done D. can’t have done
[例2]I ________to him because he phoned me shortly afterwards.
A. ought to have written B. must have written
C. couldn’t have written D. needn’t have written
[例3]You________ those letters. Why didn’t you
A. should post B. must have posted
C. should have posted D. ought to post
