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一、may/might,must,can的推测性用法
1. 意义与适用范围
may表示“或许,可能”,用于肯定句。might表示“或许,可能”,可能性比may小,用于肯定句。must表示“一定,必定”,用于肯定句。can表示“或许,可能”,用于疑问句和否定句。
例1 ―_______ the news that our headmaster has returned from abroad be true?
―No,it _______.
A. Can;cannot B. Can;mustn’t
C. May;cannot D. Must;mustn’t
解析:选A。第一空表示“可能”且用于疑问句,因此应填can;第二空表示“不可能”,因此应填cannot。
例2 Since you have so much work to do,you _______ be very busy.
A. can B. may C. must D. need
解析:选C。since you have so much work to do暗示该空表示“一定”,因此应填must。
2. 反意疑问句的构成
may,might,must,can等推测性情态动词所在句构成反意疑问句时,谓语动词不能再用情态动词,而应与其后的实义动词保持一致。其时态的使用遵循下列原则:“may / must / cannot +动词原形”,表示对现在情况的推测,反意疑问句应用一般现在时;“may / must / cannot + have done sth.+过去时间状语”,表示对过去情况的推测,反意疑问句用一般过去时;“may / must / cannot +have done sth.”的反意疑问句用现在完成时。
例3 Now,your daughter must be at home doing her homework,_______ she?
A. mustn’t
B. needn’t
C. isn’t
D. cannot
解析:选C。题干中的must表示推测,且其后接动词原形,因此反意疑问句用一般现在时。
例4 Your youngest brother may have gone to see a film last night,_______ he?
A. hasn’t B. didn’t
C. mayn’t D. doesn’t
解析:选B。题干中的may表示推测,且其后接“have done+过去时间状语”,因此反意疑问句用一般过去时。
二、 may,must,can的非推测性用法
1.意义与用法
can还可表示“能,会,有时也会,可以”,常用来征求对方意见或向对方提出请求。may还可表示“可以”,用来表示许可对方做某事、征求对方意见或向对方提出请求。must还可表示“必须,应该”,意义否定式为needn’t,表示“不必”;形式否定式为mustn’t,表示“不准,禁止”。
例5 Peter,don’t you know that even gentle girls _______ be very naughty?
A. must B. may C. can D. shall
解析:选C。该空表示“有时也会”,因此应填can。
例6 Mary,it’s already midnight. _______ you play the violin at such a late time?
A. Can B. Must C. May D. Need
解析:选B。由句中隐含的责怪、埋怨口吻可以推出,该空表示“非得”,因此应填must。
2.委婉表达
might可以代替may,could可以代替can,语气更为委婉、客气、礼貌,但对其开头的一般疑问句回答时应还原成may和can。
例7 ―Could I put you to the trouble of answering me a question?
―Yes,you _______ .
A. could B. might C. can D. must
解析:选C。could用于一般疑问句,表示客气的请求时,回答应还原成can。
例8 ―Might I have a few minutes of rest now?I’m so tired.
―Yes,you _______ .
A. might B. may C. could D. need
解析:选B。might用于一般疑问句,表示客气的请求时,回答应还原成may。
三、shall,should的用法
1. shall可表示“需要”,用来征求对方意见,用于第一、三人称;shall还可以表示“命令,警告,威胁,强制,允诺”,用于第二、三人称。
例9 Mary has got home. _______ she come here to see you immediately?
A. Will B. Shall C. May D. Can
解析:选B。该空意为“需要”,用于第三人称,表示征求意见,因此应填shall。
2.should可表示“应该,竟然”,强调惊讶的情绪;should还可以表示“如果,万一”,位于句首,构成条件状语从句。
例10 _______ a fire break out,you must sound the fire alarm in the shortest time.
A. Would B. Could C. Might D. Should
解析:选D。该空表示“如果,万一”,因此应填should。
四、“情态动词+have done sth.”的用法
1. may/might have done sth.,意为相对于现在/过去某一时刻可能已做某事;must have done sth.,意为相对于现在/过去某一时刻肯定已做某事;cannot have done sth.,意为相对于现在不可能已经做了某事。
例11 ―I have looked for him everywhere but I cannot find him.
―He _______ home.
A. can have gone B. must go
C. may go D. must have gone
解析:选D。由“I have looked for him everywhere but I cannot find him”可以推出,该空表示“一定已经回家了”,因此应填must have gone。
例12 The door in her office is still open,so she _______ very far.
A. cannot go
B. cannot have gone
C. shouldn’t have gone
D. oughtn’t to have gone
解析:选B。the door in her office is still open暗示该空表示“不可能已经走了”,因此应填cannot have gone。
2. should/ought to have done sth.,意为本来应做某事但实际未做;shouldn’t / oughtn’t to have done sth.,意为本来不应该做某事但实际已做。
例13 You are still doing your homework. You _______ it an hour and a half ago.
A. should finish B. ought to finish
C. must have finished D. should have finished
解析:选D。are still doing与an hour and a half ago暗示该空表示“本来应该完成但实际没有”,因此应填should have finished。
例14 Your father looks so sad. You _______ with him.
A. shouldn’t quarrel
B. oughtn’t to quarrel
C. shouldn’t have quarreled
D. cannot have quarreled
解析:选C。由your father looks so sad可以推出该空表示“本来不应该争吵”,因此应填shouldn’t have quarreled。
3. needn’t have done sth.,意为本来不必做某事但实际已做。
例15 The final exam turned out to be so easy,so you _______ about it.
A. cannot have worried
B. shouldn’t have worried
C. oughtn’t to have worried
D. needn’t have worried
解析:选D。 turned out to be so easy暗示该空表示“本来没有必要担心”,因此应填needn’t have worried。