解放军文职考试：情态动词表示推测的用法:

一、“情态动词+动词原形”表示对现在的推测。

　　1、can表示推测时一般用于否定句或疑问句。如：That man can't be her husbandshe is still single.

　　Who is knocking at the door?　Can it be the postman?

　　2、must表示肯定的推测，一般用于肯定句中。如：He must be in his office now.Mr Li must be working now， for the lights in his office are still on.

　　3、might 表示推测时不一定是may的过去时，只是表示其可能性较小。如：The man may be the headmaster.

　　—Where is Mr Li? —He might be working in his office. —May Mr Li come? —He might not come here.

　　4、Could表示推测时，语气can比要弱，说话者留有余地。如：—Could it be an animal?—It could not be， because it is not moving.

　　5、Should表示推测的可能性比较大，仅比must的可能性 小一点。如：It is already 10 o’clock now they should be there.

　　二、情态动词表示对过去可能发生的动作或存在过的动作的推测性用法。

　　1、“must +have done/been------”表示“过去一定发生过某事或存在过某种状态”，不用于“musn’t+have-----”形式。如：She must have seen the film before，hasn’t she?(注意反意疑问句的后半部分)

　　You must have met uncle Wang in the shop yesterday，didn’t you? (注意反意疑问句的后半部分)

　　2、“should +have done /been-------”表示“本应当做某事，而实际上并没有做”;“shouldn’t+完成式”表示“本来不应当做，而实际上却做了”。以上结构常带有说话者的责备的感情色彩。如：You should have finished your homework earlier (but in fact you did not finish it on time).

　　You shouldn’t have gone to bed when you woke up at five (but in fact you went to bed again then).

　　3、“needn’t+完成式”表示“本来没有必要做某事，而实际上却做了”。如：There was plenty of time. She needn't have hurried .

　　4、“can’t /couldn’t+have done /been-------”表示“过去不可能发生了某事或存在过某种状态”。如：

　　I saw him just now. He can’t have gone to Japan.

　　She said the man couldn’t have stolen her car.

　　5、“could+have done/been-------”表示“过去本来能够，可以做某事或成为某种状态，而实际上没有”，说话者有些遗憾。“could sb. have done /been-------?”是它的问句形式。如：You could have stayed with the Smiths while you were in New York (but in fact you stayed in a nearby hotel .) Could Mr Li have helped this girl student?

　　6、“may/might+完成式”表示“过去可能，本来可以于某事而实际上没有干”，might的可能性较小，语气较弱。如：He may have finished reading the book. She might have given you some help，however bus she was.