四、情态动词用法归类与高考试题分析：
（一）表示推测的情态动词
对现在或将来情况推测，用情态动词+do / be / be doing…；对过去的推测，用情态动词+have done / been…；表示肯定推测的情态动词按可能性大小依次为 must, should / ought to, may, might, could;否定推测，常用can't / couldn't,译为"根本不可能"、"想必不会",表示推测的语气非常肯定，may not, might not或could not译为"可能不"、"也许不",表示推测的语气不很肯定。
（1） must表示推测，意为"一定……",只能用于肯定句中。 must have done意为：一定做过某事或某事肯定发生了。 例如：
（NMET2007 江苏）-She looks very happy. She ______ have passed the exam.
-I guess so. It's not difficult after all.
A. should B. could C. must D. might
【解析】句意为：她看上去很高兴，一定是通过了考试。
【答案】C
（2） should （not） / ought （not） to在中表示根据常规或常识推测，表示"某事应该或不应该发生",语气比must或can't / couldn't稍弱。
例如： （NMET2007全国I）-How's your tour around the North Lake? Is it beautiful?
-It ________ be, but it is now heavily polluted.
A. will B. would C. should D. must
【解析】should此处表示推测，意为"应该".
【答案】C
（NMET2005上海）There _________ be any difficulty about passing the road test since you have practiced a lot in the driving school.
A. mustn't B. shan't C. shouldn't D. needn't
【解析】句意：既然你在驾校进行了大量训练，那么通过道路测试按理不应该有困难。
【答案】C
（3） can, could表达推测时，一般用于疑问句和否定句中；can用于肯定句中表示一种理论上的可能性，并不牵涉是否真的会发生，在这种用法中can只能与动词原形连用；could用于肯定句中，语气比may/ might更弱。例如：
①（NMET2008浙江）You ______ be hungry already - you had lunch only two hours ago!
A. wouldn't B. can't C. mustn't D. needn't
②（NMET2008天津）She ______ have left school, for her bike is still here.
A. can't B. wouldn't C. shouldn't D. needn't