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.
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.
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.
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
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.
2.在提建议时, 可用Can I / you… ?
a. Can I buy you a drink ?
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 ?
3.在提建议时, 可用May I … ?
a. May I carry your bag ?
b. May I make a suggestion ?
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?
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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