一、用法归纳

1. 表示推测的情态动词有must, can, could, may, might等，且语气由左向右依次减弱。could, might并不一定是can, may的过去式，有时只是语气分别比can, may更弱。

2. must (一定会，准会) 只用于肯定句；can (可能)一般用于疑问句或否定句；may (也许) 用于肯定句或否定句。虽然must和may均可用于肯定句，但在语气上must比may强得多，must表示有把握的肯定推测，而may表示没有把握的肯定推测。虽然can和may均可用于否定句，但在语气上can’t(不可能)比may not(可能不)要强得多，can’t是表示有把握的否定推测，而may not是表示没有把握的否定推测。could, might可用于肯定句、否定句或疑问句中，其区别在于语气上强弱的区分。

3. should, ought to (按理应当), would, will,(很可能)等也可表示推测，语气都较肯定，且由左至右依次增强，但较must稍弱。

4. 关于表示推测的情态动词后面所接动词形式，有下列三种情况：若对此刻正在发生的事，接“be +现在分词”；若对现在或未来情况的推测，接动词原形；若对过去情况的推测，接“have +过去分词”。

 

二、实例分析

◎—Excuse me. Is this the right way to the Summer Palace?

—Sorry, I am not sure. It ________ be. (湖北卷)

A. might                        B. will

C. must                         D. can

【分析】答案选A。由I am not sure(我没有把握)可知，后文是没有把握的肯定推测，用might。

◎You might just as well tell the manufacturer that male customers ________ not like the design of the furniture. (上海卷)  

A. must                          B. shall

C. may                           D. need

【分析】答案选C。. may not意为“可能不”。句意是：你不妨告诉厂商男顾客可能不喜欢这种家具的花样。

◎—Isn’t that Ann’s husband over there?

—No, it ________ be him—I’m sure he doesn’t wear glasses. (全国卷)

A. can’t                          B. must not

C. won’t                         D. may not

【分析】答案选A。由No和后文I’m sure…可知，是有把握的否定推测，所以用can’t (不可能)。

◎—When can I come for the photos? I need them tomorrow afternoon.

—They ________ be ready by 12:00. (全国卷)

A. can                             B. should

C. might                          D. need

【分析】答案选B。should表示“按理应当，大概”。用might语气太不肯定，顾客可能走掉。

◎I have lost one of my gloves. I ________ somewhere. (北京春)

A. must drop                       B. must have dropped           

C. must be dropping             D. must have been dropped

【分析】答案选B。对已经发生的事进行推测用“must have +过去分词”，答案D是被动式，应排除。

◎—I stayed at a hotel while in New York.

—Oh, did you? You ________ with Barbara. (全国卷)

A. could have stayed              B. could stay

C. would stay                        D. must have stayed

【分析】答案选A。表示对过去情况的推测：“你可能和Barbara住在一起了”；must语气太肯定，与前面具有疑问语气的Oh, did you?不符。

◎—There were already five people in the car but they managed to take me as well.

—It ________ a comfortable journey. (全国卷)

A. can’t be                            B. shouldn’t be

C. mustn’t have been              D. couldn’t have been

【分析】答案选D。既然小汽车上“已有五人”，且是“设法”才把你带去的，如此拥挤，旅途当然“不可能”舒服。

◎—Tom graduated from college at a very young age.

—Oh, he ________ have been a very smart boy then. (全国III)

A. could                                B. should

C. might                                D. must

【分析】答案选D。由上文可知，应是有把握的肯定推测，用must(一定)。

◎You ________ be tired—you’ve only been working for an hour. (四川卷)

A. must not                           B. won’t

C. can’t                                D. may not

【分析】答案选C。由后文“你才只工作了一个小时”推断，“你不可能就累了”。表示有把握的否定推测，用can’t。must not不允许；may not可能不。

◎It’s nearly seven o’clock. Jack ________ be here at any moment. (全国卷)

A. must                                B. need

C. should                              D. can

【分析】答案选C。nearly暗示“时间不早了”，“按理应当(should)”Jack“随时(at any moment)”都有可能到达这里。must又语气过强；can一般不用于肯定推测；need意义不通，也不用于肯定句。

◎I didn’t hear the phone. I ________ asleep. (全国卷)

A. must be                             B. must have been

C. should be                           D. should have been

【分析】答案选B。当时我没不听到电话铃响，那“一定是己经睡着了。

◎—Is John coming by train?

—He should, but he ________ not. He likes driving his car. (全国卷)

A. must                                 B. can

C. need                                 D. may

【分析】答案选D。由“他自己喜欢开车”这一习惯推断，“他可能不会(may not)坐火车来”。can not“不可能”，语气太肯定。

◎Peter ________ come with us tonight, but he isn’t very sure yet. (全国卷)

A. must                                B. can

C. may                                 D. will

【分析】答案选C。由but he isn’t very sure. (但他没有把握)可知，Peter晚上来的可能性是不大的。表示没有把握的肯定推测用may(也许)。

◎—Are you coming to Jeff’s party?

—I’m not sure. I ________ go to the concert instead. (全国卷)

A. must                               B. would

C. should                             D. might

【分析】答案选D。由I’m not sure可知，“我可能(might)要去听音乐会”

◎Michael ________ be a policeman, for he’s much too short. (上海卷)

A. needn’t                            B. can’t

C. should                             D. may

【分析】答案选B。由he’s too short推知，Michael“不可能(can’t)”是 policeman。