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?

5.can’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.

7.can / 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

三.must的用法:
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.

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.

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.would do sth可以表示过去经常发生的动作, 相当于used to do sth
a. When we were children we would go swimming every summer.
b. On Sundays he would go fishing for hours when he lived in the countryside.

3.表示请求, 愿望, 语气客气, 委婉
a. I would like some tea.
b. Would you mind closing the door ?
c. Would you tell me something about your trip ?

4.表示推测, 表示大概, 也许的意思
a. That would be the pen you are looking for.那也许是你在找的笔吧

五.should的用法:
1.表示义务, 责任, 可译为应当
a. We should complete the text in time.
b. You should be so careless.

2.表示推测或推论, 可译为可能, 应该是
a. He should be home by now, I think.
b. He should have arrived in Nanjing by this time.

3.should / ought to的区别:
should / ought to一般可以通用. should侧重于说话人主观上的看法, 有时含有劝告, 建议的口气, 即 按我的想法应该如何; ought to语气更强, 强调有责任, 有义务做某事或者 按道理应该如何
a. I will start the work at once. I think you should start at once.
b. I will start the work tomorrow. No. You ought to start at once.