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3.must / have to的区别:

   ①.must / have to一般可以通用, 但must侧重于说话人主观上的看法, 即“说话人认为必须”; have to侧重于客观上的需要, 含有“客观上不得不”之意

      a. If the person is not breathing, you must try to start his breathing.

      b. You must / have to study with a teacher if you want to know how to do first aid.

   ②.must没有时态的变化, 一般用于表示现在或将来;  have to有时态的变化, 可用于过去, 现在, 将来各种时态

      a. We must study hard when we are young.

      b. I think she must remain in hospital for a week.

      c. We had to stay there for a whole day because of the rain.

      d. We have to practise a lot if we want to speak English well.

      e. The situation has changed; we will have to change our plan.

   ③.它们的否定式mustn’t / not have to有很大的不同: mustn’t表示“不要(做某事)”, 有禁止之意; not have to表示“不必要(做某事)”, 含有“客观上无此必要”之意

      a. You mustn’t move a person if he is badly hurt.

      b. You don’t have to be a doctor to do first aid.

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2.表示“肯定是, 一定是”的推测意义; 与此对应, 表示“肯定不,一定不”用can’t, 而不用mustn’t

     a. You must be very tired now.

     b. If he had really been there, I must have seen him.

     c. He must have gone away. We don’t see him anywhere.

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1.表示“必须”, 它的否定形式是need not / needn’t, 而不是must not / mustn’t, mustn’t表示禁止或不许做某事

     a. You must set off at once.

     b. You needn’t tell John about it.

     c. You mustn’t play with fire.

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4.在疑问句中表示怀疑, 不确定或不会有的情况, 即“否定的推测”

     a. Can it be true ? 这是真的吗?

     b. Can it be true that he has passed the exam ? 他真的通过考试了吗?

 ’t / couldn’t在陈述句中可表示“肯定不, 一定不”的意思

     a. He can’t be in the room right now.

     b. It can’t have rained last night, for the ground is dry.

’t / couldn’t help doing sth表示“不得不,忍不住做某事”

     a. People couldn’t help laughing at the foolish emperor.

  / be able to do的区别:

   ①.be able to可用于各种时态, 而can只有现在时can和过去时could

   ②.be able to可以和另一个不完全动词连用, 如should be able to (应该能够), must be able to (必须能够)等, 而can则不可以这样用

   ③.强调“能力”时, 多用be able to

      a. The patient was soon able to sit up and read.

   ④.can可用于人或其他事物作主语的句子中;  be able to只用于有生命的名词或代词作主语的句子中

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3.表示“会, 能”, 相当于be able to

     a. He can speak English.

     b. Can you play tennis ?

     c. Little Tom can’t move the big box.

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2.在提建议时, 可用Can I / you… ?

     a. Can I buy you a drink ?

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1.表示许可或请求许可, 相当于may. 但can比may用得更广泛. can不仅表示说话人同意, 准许, 还可以表示客观条件许可. may通常只表示说话人同意或准许

     a. The class is over. You can go home now.

     b. You can go there tomorrow.

     c. Can I borrow your car for today ?

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3.在提建议时, 可用May I … ?

     a. May I carry your bag ?

     b. May I make a suggestion ?

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2.表示“可能, 也许” (在疑问句中通常不用may / might, 而用likely, do you think, can等表示“是否可能, 会不会”的意思)

     a. He may come today.

     b. Is it likely to rain ?

     c. Do you think the train will be late ?

     d. Can the news be true ?这消息会是真的吗?

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1.表示“可以”, 即表示说话人许可或请求许可

     a. You may take it away.

     b. May I come in ?

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