②表示“有义务或有必要”，用should或ought to。 即“应该”。 If you see anything unusual, you should call the police. ③用must 译：“必须”。
The guide required that all passengers must wear seat belts. 导游要求所有旅客必须系安全带。
2）反面的规劝，表示“不必”，“不应当”，或“不许”做某事。
①表示“不必”，用needn’t 或 don’t have to 。 --- I don't mind telling you what I know.
--- You needn’t. I'm not asking you for it. (04江苏卷)
You needn’t return / don’t have to return the book now. You can keep it till next week.
②表示“不应该”，用shouldn’t 或 oughtn’t to。
We shouldn’t throw cold water on them. 我们不应当给他们泼冷水。 ③表示“不许”,用mustn’t。
You mustn’t smoke in class. 上课时不准吸烟。
needn’t / don’t have to → shouldn’t → mustn’t 语气逐步加强 5．须区别或注意的情态动词 1）can, could与be able to的区别
a）表示能力时can和be able to可以互换使用
I can / am able to speak English. She could / was able to read novels at ten.
b）表示在过去某一特定场合下，一次性表现出来的能力，只能用was / were able to。 The fire spread through the hotel very quickly but everyone get out.
A．had to B．would C．could D．was able to（答案D）（97全国） 2）shall用在主语是第二、第三人称的陈述句中，表示说话人给对方的许诺或命令。 Work hard and you shall be paid. 好好干，你会得到报酬的。（许诺）
You shall leave here at once. = You must leave here at once. （命令）
shall也用于法律、法令或规章等正式文件中，表示义务或规定。译“应，必须”。 The fine shall be paid in cash. 罚款应以现金交纳。 3）need和dare的用法。 改错：
1）To become a member of this club, one need attend three meeting and pay his fees regularly. A B C D 2）We do need masses of young people who dare accept all kinds of challenges. A B C D
a）need（需要），dare（敢于）可作行为动词用，句型need to do, dare to do。也可作情态动词，但不能用在肯定句。在肯定句中只能作行为动词使用，故第一题B错，need → needs to；第二题C错，accept → to accept
b）need,作情态动词用时，无人称变化。need用于否定句和疑问句中；dare用于否定句、疑问句和条件句中。
You needn’t work so hard. → You don’t need to work so hard.
45
— Need I go at once.
— Yes, you must. / No, you needn’t.
Mary dare not touch the snake. → Mary doesn’t dare（to）touch the snake. Dare you walk through the forest at night ?
If the enemy dare enter our country, we will fight against them to the end. 如果敌人胆敢进入我国，我们就和他们战斗到底。 4）should的用法。
should除表示“应该”外，还可用于表示情感、意志等句中的that从句。译“竟然、居然”。例如：
You can’t imagine that a well-behaved gentleman _____ be so rude to a lady.
A. might B. need C. should D. would （答案C）（01上海）
第二节 推测
1．根据某一事实、现象推测可能发生的情况。用must, may, might。下表比较了三者表示推测肯定程度的差别，以及对不同情况推测的表达方式。 大 可 能 性 小 对当前情况的推测 must be be doing may be be doing might be be doing 对已发生情况推测 must have done may have done 对将来可能发生的情况 不能 may do might have done might do 例：You must be tired. Go home and have a good rest. （对目前的推测）
The light is on in the lab. Mr Li must still be doing experiments. （对目前的推测） Sorry I'm late. I might have turned off the alarm clock and gone back to sleep again.
（对过去的推测）（2000北京春季）
注：1）must表示推测仅用于对当前和已发生的情况，不能用于推测将来可能发生的事情。must表示推测仅用于肯定句。在否定句和疑问句中，必须用can / could取代must。 Mr Li can’t still be doing experiments at present.
My English-Chinese dictionary has disappeared．Who ___________ have taken it？
A．should B．must C．could D．would 答案C （03上海春季）
2）may / might可以用于推测将来可能发生的情况，既可用于肯定句，也可用于否定句，但不能用于疑问句。在疑问句中，同样使用can / could。 例：1. —Is John coming by train?
— He should, but he________not. He likes driving his car.
A. must B. can
C. need D. may 答案D （02全国）
2. — Your son may return to Nanjing tomorrow by way of Hong Kong. — Wonderful! But how can he come, by air or sea?
46
2．根据经验或惯例对某一情况进行推测，用should或ought to，常译成“按理应该?”或“可能会??”。
It’s nearly seven o’clock. Jack should be here at any moment. 时间已将近7点了，按理Jack该回来了。
第三节 反思
1．表示“当时本应该做，而实际上没有做”，用： ①should have done或ought to have done ②could have done或might have done
句型①责备的语气强于句型②
例：1. Mr. White should have arrived at 8:30 for the meeting, but he didn’t’ show up.
(04广西卷)
2.He might have given you more help, even though he was busy.
2．表示“当时没有必要”做某事
①needn’t have done 当时做了某事，但事实上没有必要 You needn’t have told him the news; he knew it already.
didn’t have to do 通常指当时某事没有必要做且没有做。 She told me that she had a daughter and didn’t have to have another baby.
第四节 虚拟语气
虚拟语气是指所说的话只是一个与事实相反的主观愿望或假想。虚拟语气可用于条件状语从句、名词从句以及其他结构中。 一、if引导的非真实条件中的虚拟语气。 1、三种形式。具体情况见下表： 假设类型 与现在事实相反 与过去事实相反 将来不可能发生 if条件句谓语动词形式 主句谓语动词形式 形) had done were to do或should do would / could / might have done would / could / might + do(动词原形) 例：1. If there were no gravity, we would not be able to walk on the earth.
假如没有引力，我们就不可能在地球上行走。
2. We would have succeeded if we had made enough preparations. 当初要是作了充分的准备，我们就有可能成功。
3. This kind of boiler is very safe. If it were to go wrong, the controlling system would cut off the fuel oil supply automatically.
这种锅炉很安全。万一锅炉发生故障，控制系统会自动切断燃油的供给。
2、if虚拟条件句的省略。
47
动词用过去时形式（be用were） would / could / might + do(动词原若虚拟条件从句中有were, had, should, 可把if省略，而把were, had, should提到主语前面，成为倒装结构。
例：1.If I were you, I would accept the challenge. → Were I you, I would accept the challenge. 2. If you had informed me earlier, I wouldn’t have signed the contract.
→Had you informed me earlier, I wouldn’t have signed the contract.
当初你要早点告诉我，我是不会签那个合同的。 3、错综时间条件句。
在这种虚拟条件句中，从句动作和主举动作发生的时间不一致，因此，从句和主句的谓语动词要根据各自所指的不同时间选择适当的动词形式。
例：1. If I were you, I would have taken that job. （从句指现在，主句指过去）
你错过了这么好的机会。我要是你，我当时就接受那份工作。
2. Had her husband not been killed in the war, she would not be helping others with housework
now. （从句指过去，主句指现在）
如果她的丈夫那时没有死与战争的话，她现在不会帮人做家务了。 4、含蓄虚拟条件句。
有时假设的情况并不以条件从句表示出来，而是以其他形式表现出来。
1）介词短语but for 或 without。译：“要不是，要是没有”。
例：1. But for electricity (= If it were not for electricity), there would be no modern industry. 要是没有电，就不会有现代化工业。
2. Without your help (If it had not been for your help), we could not have succeeded. 当初没有您的帮助，我们不可能成功。
2）or 或otherwise。句型：did(过去时)，or / otherwise would have done. 例：I ran all the way to school, otherwise I would have been late. 我一路跑到学校，否则，我就会迟到了。
3）but句型：（1）would do but do (一般现在时)
（2）would have done but did (一般过去时)
My daughter would gain weight but she doesn’t eat enough. ---- Why didn’t you help him out? ---- I would have but I didn’t have money.
5、wish后的宾语从句和if only引导的感叹句中的虚拟语气。
wish后的宾语从句和if only引导的感叹句表达的是与客观事实相反的愿望，需用虚拟语气。
wish that从句 = if only + 从句 译：“要是??就好了” 或 “但愿??”。 1）愿望与事实相反。从句谓语动词用：were或did (过去时形式)
I wish I knew the answer to the question. = If only I knew the answer to the question. 我要能知道那个问题的答案就好了。
2）愿望与过去事实相反。从句谓语动词用：had done (过去完成时形式)
---- We had a great time last night.
---- I wish that I had not been ill yesterday. = If only I had not been ill yesterday. 昨天我要是不生病那该多好啊！
48