神态动词现在完成式主要有两个功用：表明现已发作的状况和表明虚拟语气。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等神态动词＋完成式表明的意思是有必定差异的。

1.表明现已发作的状况

1）must have+过去分词，表明对已发作状况的必定估测，译为“（昨日）必定……”。

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，表明对已发作的工作做不必定、可能性很小的估测，或事实上底子没发作，译为“或许……”。如：

At Florida Power’s Crystal River plant， a potentially serious leakage of radioactive water may have been unknowingly caused by an electrician.

2.表明虚拟语气。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，实践成绩是否真的穿的很正式没有确认，答案为D）

2）should have +过去分词，表明应该做某事但实践上未做，译为“本应该……”should not + have过去分词表明本不应该做某事但实践上做了，译为“本不应该……”。

3） ought to have +曩昔分词，表明动作按理该发作了，但实践上未发作，译为“该……”，与should 的完成式意义类似。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 +曩昔分词，表明曩昔原本能够做但却未做，译为“完全能够……”。这点与ought/should/ have +曩昔分词用法类似。如：

What you said is right， but you could have phrased it more tactfully.

5） may/ might have +曩昔分词，表明曩昔能够做但实践未做，译为“（那样）或许会……”