must只用于肯定句中。 can/could不能用语肯定句中。 may/might不能用语疑问句中。 另外还应注意can’t表示"不可能"，may not表示"可能不"。 考试时，如果有原因说明自己的猜测，一般应该must, can’t用而不是may, may not。如： He can’t have stolen the money. he is not such kind of person. Some pleasant thing must have happened to him. He is so excited. 3） 注意推测性用法中谓语动词发生的时间。 如果推测的为过去的事情，情态动词后则用完成形式。 There must have been no one in, for nobody answered the phone. 如果推测的是正在进行的事情，情态动词后则用动词的进行形式。 They must be talking about something very secret. 如果推测的是现在的状态等，情态动词后则用原形动词 He must be badly ill. He looks so pale. 如果推测的是将来发生的事情，情态动词后同样用原形动词，这时，情态动词只能是may/might。 There may be a terrible storm in the following few days. 4） 注意"情态动词 动词的完成形式"所表达的意思与题干意思是否相符。 其它"情态动词 动词的完成形式"并不表示推测，而分别表示： could have done本来能够 He didn’t take part in the competition, he _______ though. A. won B. didn’t win C. could win D. could have won needn’t have done 本来没不要 You ________. There was plenty of time. A. needn’t hurry B. can’t hurry C. mustn’t have hurried D. needn’t have hurried should/ought to have done 本来应该 You _______ me earlier. I could have helped you. A. should tell me B. should have told C. need to tell D. needn’t have told shouldn’t/oughtn’t to have done 本来不应该 You ____ him so closely. you should have kept your distance. A. shouldn’t follow B. mustn’t follow (2000.6) C. couldn’t have been following D. shouldn’t have been following might have done 本来应该（用来表示一种责备） You ______ even though you were busy at that time. A. might help him B. must have helped him C. might have helped him D. should help him 这类题在设计干扰项时，一般都会有情态动词推测性用法的涉及，同学们应注意分词上下文的逻辑和语意关系。 5） 注意常用情态动词的基本句法和表意功能 其它常用动词的用法，如shall表示"命令"、"威胁"、"许诺"或征求许可，may/might as well等，同学们都应多多注意其用法。