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 / neednt, 而不是must not / mustnt, mustnt表示禁止或不许做某事
a. You must set off at once.
b. You neednt tell John about it.
c. You mustnt 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cant, 而不用mustnt
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 dont 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.
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