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/cann’t；need/needn’t；may/mayn’t；might/mightn’t；should/shouldn’t；ougtht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 asked sympathetically：“ 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……”。
Mary __________ my letter； otherwise she would have replied before now.
[A] couldn’t have received [B] ought to have received
[C] has received [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 Florida Power’s Crystal River plant， a potentially serious leakage of radioactive water may have been unknowingly caused 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’t have come over yourself.
As it turned out to be a small house party， we __________ so formally.
[A] needn’t dress up [B]did not need have dressed up
[C] did not need 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 regret having left the work unfinished； I should 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：
Ｔhe porter ought to have called the fire-brigade as soon as he saw the fire in the stock， 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 said is 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 have been better to include more punchy statistics and photos of equipment in the introduction to further assist first-time office automation managers.
