1．should/ought to＋have done，表示“过去本应该做而(实际)没有做的事情”，含有责备或遗憾的语气，意为“本应该……”。其否定形式为“should not/ought not to have＋done”，表示某种行为本不该发生但实际上发生了。
I shouldn't have watched that movie—it'll give me horrible dreams. 我本不应该看那部电影——它会令我做噩梦。
You ought to have come to the party yesterday, but why didn't you come? 昨天你本来应该参加聚会的，可是你为什么不来？
2．must＋have done，用于肯定句，表示“过去一定做过某事”，表示一种很有把握的推测。注意，对过去发生的情况的否定推测常用“can't/couldn't＋have done”表示。
—Ye Shiwen won two gold medals in London Olympic Games. —She must have gone through tough training. ——叶诗文在伦敦奥运会获得了两枚金牌。 ——她肯定受到严格的训练。
—Do you know where David is? I couldn't find him anywhere. —Well. He can't have gone far—his coat's still here. ——你知道大卫在哪里吗？我到处找不到他。 ——大卫的上衣还在这里，因此他肯定没走远。
3．needn't＋have done，表示“过去没有必要做某事，但实际上做了某事”。 Mark needn't have hurried. After driving at top speed, he arrived half an hour early. 马克本没必要那么匆忙。他以最快速度开车，结果早到了半个小时。
4．may/might＋have done，表示对过去情况的一种不太有把握的可能性的推测，表示“可能已做了某事……”，否定句表示“可能还没有……”。
Sorry, I'm late. I might have turned off the alarm clock and gone back to sleep again. 对不起，我迟到了。我可能把闹钟关掉后又睡着了。
5．could＋have done表示“过去本来可以做某事，但实际上没有做”；can/could＋have done表示“过去可能做了某件事”。
I could have saved the poor rabbit, but I didn't have the right drugs with me at that moment. 我本来可以拯救那只可怜的兔子的，可是我当时没有合适的药品。 第二部分、虚拟语气
一、虚拟语气在虚拟条件句中的用法
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虚拟情况 从句谓语 主句谓语 would/ 例句 与现在事 实相反 过去式 should/ (be用were) could/might do would/ If he were here, he might be able to help. What would you do if you were in his place？ If I had started a little earlier, I would have caught the train. I could have done it better if I had been more careful. 与过去事had done 实相反 should/ could/might have done would/ 过去式 与将来事were to do/ 实相反 should do should/ could/might do If I were to do it, I would do it in a different way. I would certainly go if I had time. 1.在具体运用中，条件从句中有时可省略if而采用倒装结构。如： Had it not been for your help, we wouldn't have achieved so much.
2．介词without/but for、连词but、副词otherwise常用来表示某种假设条件。如： I wouldn't have made such rapid progress without your help.
3．有时候从句动作和主句动作发生的时间不一致，这时谓语动词的形式要根据各自的时间来调整。如： If the weather had been more favourable, the crops would be growing still better. 二、“should＋动词原形”表示的虚拟语气
这一类型常见词有“一坚持(insist)、二命令(order, command)、三建议(advise, suggest, propose)、四要求(request, require, demand, desire)”。以上词及其派生名词所接的名词性从句都使用“(should＋)动词原形”的虚拟语气。
①Jane's pale face suggested that she was ill, and her mother suggested that she(should) have a medical examination.
简苍白的脸色表明她病了，她母亲建议她去检查身体。
②She insisted that the man had stolen her car and that he(should) be sent to prison. 她坚持说那个男人偷了她的车，并坚决要求他坐牢。 三、特殊句式中的虚拟语气 If only/It's (high) time (that)．..
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wish/would rather/as if等固定句式要求用相应的虚拟语气表达形式。 ①I wish I could fly. 真希望我能飞。
②I would rather you came tomorrow. 我宁愿你明天来。
③If only I had taken your advice! 要是听从了你的建议该多好啊！
高频考点一、考查情态动词
例1．（2018·北京卷）In today’s information age, the loss of data _________ cause serious problems for a company.
A. need B. should C. can D. must 【答案】C
【解析】考查情态动词。句意：在今天的信息时代，数据的丢失有时会对一个公司造成严重的问题。数据丢失造成严重问题是客观上会发生的情况，即“客观可能性”，故该空应用情态动词can。C选项正确。其余情态动词均没有该用法。need需要；should应该，竟然；must必须，肯定。
【变式探究】—Can’t you stay a little longer?
—It’s getting late. I really _____go now, My daughter is home alone. A .may B .can C . must D .dare 【答案】C
【解析】句意：——你不能再多待会儿吗?——很晚了，我必须得走了。我女儿一个人在家。A. may可能，可以，也许；B. can能，会；C. must必须，必然要，必定会；D. dare敢，胆敢。结合句意，故选C。
【变式探究】________ I have a word with you? It won’t take long. A. Can C. Shall 【答案】A
【解析】本题考察的是情态动词基本意义辨析。Can能够，可能；must必须，一定；非得；shall将要；should应该；句意：我可以和你谈谈吗？不会花很多时间的。根据句意可知本题使用can I…？表示询问对方是否允许。如：Can I use your dictionary?我可以使用你的字典吗？故A正确。
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B. Must D. Should
高频考点二、考查情态动词的用法
例2. （2018·天津卷） I can't find my purse. I___________ it in the supermarket yesterday, but I'm not sure. A. should leave B. must have left C. might leave D. could have left 【答案】D
【解析】考查情态动词表推测。句意：我不能找到我的钱包了，昨天我有可能把它落到超市了，但我不确定。根据句中时间状语yesterday可知，是对过去事情的推测，故用情态动词+ have done，再根据后句but I’m not sure.可知，此推测为不能肯定的推测，故用情态动词could，表示“可能”。故选D。
【变式探究】（2017?天津）My room is a mess, but I __________clean it before I go out tonight. I can do it in the morning.
A. daren’t B. shouldn’t C. needn’t D. mustn’t 【答案】C
【解析】 句意：我的房间很乱，但是在今晚我出去之前不需要打扫，我可以明天早晨再打扫。A.不敢；B.不应该；C不需要；D.不许。根据句意，故选C。
【变式探究】Why didn't you tell me about your trouble last week？If you ________(tell)me，I could have helped.
【答案】had told
【解析】句意：上个星期你为什么不告诉我你的麻烦事？如果你告诉我了，我就能帮你。根据句意和相关信息判断，条件句表示与过去事实相反的假设，此时从句用过去完成时，主句谓语用“情态动词＋have done”形式。
【变式探究】I ______ have worried before I came to the new school, for my classmates here are very friendly to me.
A. mightn’t 【答案】C
【解析】句意：我来新学校之前本没有必要担心，因为在这里我的同学对我非常友好。needn’t have done本没有必要做某事，表示实际上已经做了某事。根据句意可知选C。
高频考点三、考查虚拟语气
(2019·例3．天津卷)The workers were not better organized, otherwise they ________ the task in half the time. A．accomplished B．had accomplished C．would accomplish D．would have accomplished
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B. mustn’t C. needn’t D. couldn’t
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