情态动词现在完成式主要有两个功能：表示已经发生的情况和表示虚拟语气。在这两个方面must/mustn’t；
can/can’t；need/needn’t；may/mayn’t；might/mightn’t；should/shouldn’t；
ought等情态动词＋完成式表示的意思是有一定区别的。
1.表示已经发生的情况。
1）must have+过去分词，表示对已发生情况的肯定推测，译为“（昨天）一定……”。如：
My pain__________ apparent the moment I walked into the room， for the first man I met askedsympathetically：“Are you feeling all right？”
[A] must be[B] had been [C] must have been [D] had to be
（答案为C）
2）can’t / couldn’t have+过去分词，表示对已发生情况的否定推测，译为“（昨天）一定没……”。如：
Mary__________ my letter；otherwise she would have replied before now.
[A] couldn’thave received [B] ought to have received
[C] hasreceived [D] shouldn’t have received
（答案为A）
3）may / might have +过去分词，表示对已发生的事情做不肯定、可能性很小的推测，或事实上根本没发生，译为“也许……”。如：
At FloridaPower’s Crystal River plant， apotentially serious leakage of radioactive water may have been unknowinglycaused by an electrician.
2.表示虚拟语气。
1） needn’t have + 过去分词，表示做了不必做的事，相当于“didn’t need to do”，译为“其实没必要……”。如：
You needn’thave come over yourself.
As it turnedout to be a small house party， we__________ so formally.
[A] needn’tdress up [B]did not need have dressed up
[C] did notneed dress up [D] needn’t have dressed up
（没有必要穿的那么正式，体现是说话者的建议，实际结果是否真的穿的很正式没有确定，答案为D）
2）should have +过去分词，表示应该做某事但实际上未做，译为“本应该……”should not + have过去分词表示本不应该做某事但实际上做了，译为“本不应该……”。如：
I regrethaving left the work unfinished； Ishould have planned everything ahead carefully.
我本来应该事先认真地把每件事情规划的很好，但实际上作者还是没有规划好，以至工作没有完成。
3） ought to have +过去分词，表示动作按理该发生了，但实际上未发生，译为“该……”，与should 的完成式含义类似。如：
Ｔheporter ought to have called the fire-brigade as soon as he saw the fire in thestock，which went up in smoke .
4） could have +过去分词，表示过去本来可以做但却未做，译为“完全可以……”。这点与ought/should/ have +过去分词用法相似。如：
What you saidis right，but you could have phrased it more tactfully.
5） may/ might have +过去分词，表示过去可以做但实际未做，译为“（那样）也许会……”。如：
It might havebeen better to include more punchy statistics and photos of equipment in theintroduction to further assist first-time office automation managers.
