B.关于情态动词后面接完成式的情况
can后面接完成式 表示可能已经做了某事。例如：
He can't have left home because the television is on .
因为电视还开着所以他不会离开家了。
Where can they have gone ? 他们会去了哪儿呢？
You can't have finished the novel so soon .
你不可能这么快就看完了那本小说。
could后面接完成式 表示本可能发生或本可以做的事而没有发生或没有做；它的否定形式则表示本不可能发生或本不可以做的事而已经发生或已经做了。例如：
The output could have been increased by 15% .产量本可以提高百分之十五的。
How could he have forgotten him birthday ? 他怎么会忘了自己的生日呢？
Judging from her appearance and manner,she couldn't have been over fifty years old . 从她的仪表和举止来看，她不可能已有五十多岁了。
may后面接完成式 表示可能已经发生的动作。例如：
He may have had some training in Chinese Gongfu .他可能学过一些中国武术。
You may have narrowly missed her on your way to here.
你可能在来这儿的路上刚好与她错过了。
They may have seen that movie.他们或许看过那部电影。
might后面接完成式 表示本可能发生或本可以做的事而没有发生或没有做。例如：
They might have finished their work by now .他们现在或许已经完成任务。
She might have done better than that .你本可以做得比这更好一些的。
You might have learned more from him . 你本可以向他学到更多一些东西的。
must后面接完成式 表示想必已经发生的事。例如：
You must have talked about it with her before hand .你一定事先与她谈过这件事。
It must have snowed last night for it's so cold this morning .
今天早晨这么冷，昨晚一定下雪了。
ought to后面接完成式表示本应该发生或本应该做的事而没有发生或没有做；它的否定形式则表示本不应该发生或本不应该做的事而已经发生或已经做了。例如：
They ought to have completed their work much earlier .
他们本应该在更早一些的时候完成他们的工作的。
You oughtn't to have left your keys in the office when you went out .
你不该出去的时候将钥匙留在办公室里。
I ought to have said it to her in a calmer mood .
我应该以更平静一些的语气对她说那件事的。
need后面接完成式主要用于否定句中，表示不必要做的事而已经做了。例如：
We needn't have read the Fast Reading materials before we do it in class .
我们在做快速阅读练习之前没有必要把阅读材料先看过了。
You needn't have apologized to him for nothing yesterday .
你昨天没有必要无为地向他道歉.
should后面接完成式表示本应该发生或本应该做的事而没有发生或没有做；它的否定形式则表示本不应该发生或本不应该做的事而已经发生或已经做了。例如：
I should have written to you earlier. But I've really been busy .
我本应该早就给你写信的；不过我真的一直都很忙。
You shouldn't have left your son alone at home .
你不该将你儿子一个人留在家里的。
He shouldn't have broken the red light.
他不该闯红灯的。
