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.
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.
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