英语语法第17讲-助动词PPT

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may not重音落在助动词上，表示 “不可能”，重音落在否定词上，表 示“不许可” , 比较：
•He may not go tomorrow. •He may not go tomorrow.
•所以在书面语中，表示“不可能” 常用can’t
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can/could表示现在的可能，口气上 could比较委婉
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将来“能力”：
•通常 will/shall be able to
• I’ll be able to speak German in another two months
•具体某事：can
• Can you go and see him tomorrow morning? • If I can, I will do it.
tickets.
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如果是推测过去的事态，则用must +have done
•You must have left your handbag in the theatre.
表示推测的must的否定形式是can’t不是must not
•If Fred didn’t leave home before five, he can’t be there yet.
•You should/ought to drink less.
Should/ought to +have done:本应该……
•You should/ought to have asked my permission first.
Must表示敦促或命令，必须。
•I really must stop smoking.
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过去“能力”: could, was/were
able to
肯定句：
•Could：一般能力
• She could play the piano when she was only six.
• was/were able to(及managed to)：具体 事件
• He was able to translate the article 5
❖That’s not mine. Whose can it be? ❖It could be John’s.
can常用于否定句和疑问句，而 could不受此限。
❖If you don’t have a guide, you could lose your way. ❖It can’t/couldn’t be true. ❖Can/could the news be true?
与needn’t相当的动词形式是don’t need to/don’t
have to/haven’t gt toA: Must you leave soon?
B: No, I needn’t/don’t need to/don’t have to/haven’t
got to.
必须：must与have to
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b)表示“推测”，除用should, ought to,和 must外，还可用will/would（语气仅次于 must）。
•They should/ought to be home by now.(他们现在该到 家了) •They would be home by now.(他们现在大概到家了) •They will be home by now.（我估计他们现在一定到家 了） •They must be home by now.（他们现在一定到家了）
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表示将来的“必须”，常用have to的一定 形式（will/shall have to) 比较：
•We must do it again.(表示现在)
•We’ll have to do it again.(表示将来)
•表示过去的“必须”，常用had to
•I had to leave at six yesterday.
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在肯定陈述句中，may与can的比较：
Can指逻辑上的可能性，may指事实上 的可能性
• The road can be blocked.(按逻辑可能会堵，)
•The road may be blocked.(可能现在正堵着，事 实上)
• Mr Reed is in poor health. He can be ill at any time.
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b)表示“可能”，可用may, might, can, could
may/might：
1.既可以表示现在的可能，也可表示将来 的可能，might在口气上比may更委婉
2.现在：It may/might be true. 3.将来：He may/might leave tomorrow.
2.在疑问句中，用can，不用may。 • Can they have missed the bus? • Yes, they may have.
• Mr Reed looks pale. He may be ill.
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“may/might +have done/have been doing”,“can/could +have done”表示过去的可 能。
❖He may/might have missed the train.
❖She may/might have been expressing her true feelings at that moment.
without a dictionary.
• I talked a long time, and in the end managed to make her believe me. 在否定句中，could既可以表示过去一般能 力，也可表示过去某具体事情的能力. • I couldn’t/wasn’t able to play the piano when I was at school.
过去的不许可：He was not allowed to go.
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3表示“义务”和“必然”
a)表示“义务”，可用should, ought to,
must. 表示要求，命令时，语气由ought to、
should、must渐强
Should/ought to表示“应该”，带有敦促，劝说 之意，可交替使用，前者语气更强。
must侧重于说话人的主观意志
have to侧重客观需要
• He must say it in English. (I want him to do so.) •He have to say it in English. (He doesn’t know Chinese.)
在特定语境中，用will也可表示“义务”，常用
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will/would表示“推测”可有三种情况
1）对特定事态的推测
A: Who’s that man over there? B: That will be George, no doubt. C: That would be George, I except.
2）对某些习惯性事态的推测，will指现在习 惯，would指过去习惯
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现在“能力”：can, be able to
• As I have got enough money, I can/am able to help her.
Can:
1.某桩具体事情：eg.I can climb this cliff. 2.泛指一般的“能力”： 3. He can drive a car. 4.She can’t play the piano.
must有两种否定形式:第一个是must not,表 示“不许”、“禁止”,相应的半助动词是 be to的否定形式be not to
•You mustn’t talk like that. •You are not to talk like that.(不许你如此讲话。) 15
第二个否定形式是needn’t,表示不必。
于第二人称主语，这实际上是强化的祈使句。
•You will wait outside the gate.
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•Brian! You will close the door.
b)表示“必然”，可用should，ought to和 must(一般用于陈述句)，其中must口气最为 肯定。都指说话人根据一定情况做出推测或 判断.
•She should be here in a minute
•These young trees ought to provide shade in ten years .
•All men must die.
ought to(表示“推测”）的否定形式是 oughtn’t to
•You oughtn’t to have any difficulty getting the
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4表示“预见”和“推测”
a)表示“预见”，即表示“单纯将来”，可用 will/shall+不定式，shall用于第一人称，will用于 第二、三人称。现在一般will可用于一切人称表 “将来”
•I will be a different person when I live in England.
❖He will work all day without a rest.
❖Before his retirement he would catch the early bus every morning.
3）对某些不受时限的客观过程的推测，只用 will,表示现在时间，可与一般现在时的类似用 法交替使用。
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b)表示“不许”可用may not或者cannot
•Can I go out for a moment? •No, you can’t. •May I use your car for a few days? •No, you may not.