情态动词有can(能),may(可以),must(必须),have to(不得不),ought to(应该),dare(敢),used to(过去经常),had better(最好),would rather(宁愿)。在肯定句中它们后边都要接 动词原形 。在否定句中，can,may,must和dare后边加not；have to和ought to分别在have和ought后加not；used to的否定式可用used not to，也可用didn't use to，但后者用得较多；had better和would rather的否定式分别是had better not和would rather not。这些情态动词与现在完成时连用在某种意义上有一定的虚拟性。
I 肯定句和否定句中的情态动词
一、情态动词的现在式在肯定句中的比较
1. can表示体力或脑力方面的“能力”、“技能”或根据客观条件能做某种动作的“可能性”
1) Mild forms of execrise can some of the loss of flexibility that accompanies aging.
［A］ stop
［B］ to stop
［C］ stopping
［D］ be stopped
但表示人体力或智力的具体动作时须用 be able to
He was able to do that without any help.他不需要任何帮助就能完成这项工作。
2. may表示“允许，可以”，相当于be allowed to
2) If there is social or political change in a region ［A］ where a standard language ［B］ is spoken, local varieties ［C］ of the language may developing ［D］ .
may或might可和as well连用，表示“建议”，译为“还是……的为好”
You may as well keep a certain distance from that mad man. 你们还是离那疯子远点为好。
You might as well go home now.你还是现在回家为好。
3. must表示“必须”或“应当”、“一定”
3) The formation of snow must be occurring ［A］ slowly ［B］ ， in calm air， and at a temperature near ［C］ the freezing point ［D］ .
4. have to 加动词原形，表示“不得不”，“必须”，它比must更强调客观
Tom had to work into the deep night everyday to earn a living. 汤姆为了生计每天都得工作到深夜。
5. should 表示“劝告”，“建议”或“义务”时，译作“应当”，或表示“预测”和“可能”
He should take care of his parents as they are old enough not to live on themselves. 由于父母亲老了，不能自理，他应当照顾他们。
He should be there now. 他可能到了。
should have done在虚拟语气中表示“责备或后悔”［参见第三章第一节、二、2.］。
6. ought ，只有一种形式，即ought后必须加to，然后跟接动词原形表示“有义务”或“必要”做某事，译为“应当，应该”
4) The traditional goal of science is to discover how things are,not how they ought .
［A］ to
［B］ to be
［C］ be
［D］ have been
5) You are quite right;I am inferring in my comments ［A］ that McGraw had not ought to ［B］ have broken ［C］ in the room without his permission ［D］ .
7. dare 可以用作情态动词，后面跟不带to的动词不定式，这主要用于否定句中，它本身可有现在时第三人称单数，词尾加s，它还可以有ING分词形式(daring)和过去式及ED分词形式(dared)
6) Although Oriental ideas of woman’s subordination to man prevailed in those days,she meet with men on an equal basis.
［A］ did not dared
［B］ dared not
［C］ dared not to
［D］ did dare not to
二、情态动词在一般时否定句中的用法
can't(can not, cannot) 表示“不可能”， may not 表示“不可以”， mustn't(must not) 表示“一定不要”，“不许可”，needn't (need not) 表示“不必”，dare not ＋动词原形 表示“不敢”
He can't finish his essay by this time. 现在他不可能写完论文。
He may not sleep now. 他或许现在没在睡觉。
You mustn't criticize her in that way. 你不应那样批评她。
You needn't come tomorrow. 你明天没必要来了。
He dared not meet his fiancée. 他不敢见女朋友。
三、例题解析
1) 正确答案为A。由于情态动词can要求跟动词原形，所以B和C都不对，can后虽然有被动形式，但在意义上和语法上与横线后部分都无法衔接，所以D也错。
2) D错。改为develop。may后要求跟动词原形，而developing是现在分词，显然不符合要求，所以应改为develop。
3) A错。改用 must occur，此处叙说的是客观现象，而非强调正在发生的事，故用一般现在时。
4) B为正确答案。
5) B错。改为ought not to。
6) B为正确答案。
II 情态动词与完成时的使用
一、must+have+ED分词：用于肯定句，表示对过去情况的一种肯定推测，表示“肯定，一定”
1) It around nine o’clock when I drove back home because it was already dark.
［A］ had to be
［B］ must have been
［C］ was to be
［D］ must be
2) John’s score on the test is the highest in the class;
［A］ he should study last night
［B］ he should have studied last night
［C］ he must have studied last night
［D］ he must had to study last night
3) After searching ［A］ for evidence in the house,the police concluded ［B］ that the thief must have come in ［C］ through the window and stole ［D］ the silver while the family was asleep.
二、may (might)+have+ED分词：用于肯定句和否定句，表示对已发生事情的不肯定的推测，相当于“可能，大概，”其中might较may 语气更弱，把握更小
She might have gone to see her doctor last week, but I am not sure.上星期或许她去看医生了，但我不敢肯定。
Don’t worry， your husband may not have been hurt seriously. 别急，你丈夫也许伤得不厉害。
三、should(ought to)+have+ED分词：肯定句表示过去本应发生的事却没有发生；否定句表示已发生了本不该发生的事。前者可译为“本应，”后者为“本不该”
You should have apologized to her for not soon replying to the letter. 你本应向她道歉，说明为什么没能及时回信。(可你没这么做)
4) You yesterday if you were really serious about your work.
［A］ ought to come
［B］ ought to be coming
［C］ ought to have come
［D］ ought have come
四、can(not) +have+ED分词
He is an hour late —— He can have been delayed by fog. Of course,that’s a possibility.他迟到了1小时，可能因为大雾而耽搁了。当然这只是可能性问题。
The poem can not have been written by her since she was only five years old then.这诗不可能是她写的，因为她那时才5岁。
五、“could+have+ED分词”有时用于表示过去的时间，说明某事可能或不可能已发生；有时可表示过去本来可以做某事，但却未做
I simply can’t understand how he could have made such a mistake. 我简直不明白他怎么会犯那样的错误。
He walked there,but he could have taken a taxi. 他走着去了，可当时完全可以坐出租。
“couldn’t+have+ED分词”还表示无论如何也不可能或没有做到
I couldn’t have called you. I wasn’t near a telephone. 反正我也不可能给你打电话，我附近没有电话。
5) “We didn’t see him at the lecture yesterday.”“He it.”
［A］ mustn’t attended
［B］ couldn't have attended
［C］ would have not attended
［D］ needn’t have attended
六、needn’t+have+ED分词：表示对过去不必做的事情却做了，可译为“其实不必”
6) You all these parcels yourself.The shop would have delivered them if you had asked a shop assistant.
［A］ didn’t need to carry
［B］ needn’t have carried
［C］ needn’t carry
［D］ didn’t need carry
七、used to 表示过去的习惯动作或状态，现在已不存在，在否定陈述句中，一般用didn't use to，也可用used not to；在疑问句中用“Did…use to”。used to还可与never,often,always等连用。注意used to与be used to的不同，后者表示be accustomed to “习惯于某种状态”，而且跟接名词或ING形式，而used to后接动词原形，试比较
He used to drink. 表示“他过去经常喝酒”而现在不喝了。
He is used to wine (He is used to drinking wine) at each meal. 表示“他现在已养成习惯，每顿饭喝点葡萄酒。”
八、had better表示“最好……”，后接动词原形，其否定式为had better not＋动词原形
You had better not follow his behavior. 你最好不要学他。
九、would rather 意思是“宁愿”，表示选择。它有两种用法，
一是在肯定句中后边直接跟动词原形，其否定句是在would rather 后加not,即 would rather not＋动词原形；
二是它后边可以跟接从句，该从句的谓语动词用一般过去式表示虚拟语气。由于would rather表示选择，它后边可跟接than
She would rather sleep than talk rubbish. 她宁愿睡不愿闲聊。
7) “Did you criticize him for his mistakes?”——“Yes,but it.”
［A］ I’d rather not do
［B］ I’d rather not doing
［C］ I’d rather not have done
［D］ I’d rather not did
［注］ would rather和had rather都表示“宁愿”，在用法上也没有区别。但如果说would rather与had rather完全一样(Longman Dictionary of Comtemporary English 如是说)，这一说法有争议。其实had rather在现代标准英语中几乎已经不用了。这对我们考试来说是比较重要的，因为一般考题作为标准英语是不用had rather的。
十、例题解析
1) B为正确答案。
2) C为正确答案。
3) D错。改为stolen。这里C处和D处是并列的谓语，D处相当于must have stolen。
4) C为正确答案。
5) B为正确答案。
6) B为正确答案。
7) C为正确答案。
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