could和 must和 couldn't 的区别和用法?

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could和 must和 couldn't 的区别和用法?
could和 must和 couldn't 的区别和用法?

could和 must和 couldn't 的区别和用法?
1．三者均可表示推测,couldn’t 是could的否定式（注意,表推测时,它们并不是过去式,只是在语气上比can或can’t委婉）：
He could be i11. 他可能生病了.
He couldn’t be i11. 他不可能生病了.
He could have left. 他可能已经走了.
He couldn’t have left. 他不可能已经走了.
2. could和couldn’t 表示推测时,可用于肯定句、否定句和疑问句.如：
You could be right. 你可能是对的.
Could it have been him? 那可能是他吗?
What could it be? 它会是什么呢?
Do you think she could be lying? 你认为她可能在说谎吗?
His theory could be understood by a child. 他的理论小孩子也能理解.
3. must表示推测时,通常只用于肯定句,而不用于否定句或疑问句.如：
He must have left early 他一定走得很早.
You must have seen him. 你一定见过他了.
He must be coming by bus. 他一定是乘公共汽车来.
It must be eleven o’clock now. 现在一定是11点了.
You must be joking. 你一定是在开玩笑吧.
That hat must be Tom’s. 那帽子一定是汤姆的.
There must be something wrong. 一定有什么毛病了.

could 是 can 的过去式
换句话说就是 can 和 must 的区别
can 是能够的意思
must 是必需的意思
couldn't 是 could 的否定式
同样 must 也有是 mustn't
不过mustn't 不常用

could 和 must 为肯定 couldn't 为否定
could 较must 语气较弱

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