如果是mustn't可以用于否定句（疑问句中吗?）如果可以与couldn't,can't有什么区别吗?

来源：学生作业帮编辑：神马作文网作业帮分类：英语作业时间：2024/11/12 19:53:29

如果是mustn't可以用于否定句（疑问句中吗?）如果可以与couldn't,can't有什么区别吗?
同上.另外can与could可否用在肯定句中呢?
（当以上情态动词表示一种推测时）

1. mustn't在英语中不能用于表示推测,但是可以表示“禁止/不允许”之意.
如：You mustn't smoke here. 在这里你不许抽烟.
2. can't/couldn't表示推测时,意思是“不可能...”,语气非常坚决的否定推测.
根据动作发生的时间,一般分为以下三种情况：
1）进行式（对正在发生的事情的否定推测）：can't be doing sth
如：It's already ten o'clock. My father can't be sleeping now.
2）完成式（对过去的动作的否定推测）：can't/couldn't have done sth
如：He is at schoool, so he can't/couldn't have stayed up last night.
3）一般式（对现在/将来情况的否定推测）：can't do sth
如：Your answer can't be true.
3. can/could表示推测时一般只用于否定句/疑问句,但是也可用于肯定句,只是比较少见.
其中could不表示过去式,而表示可能性较小.
如：I think the work can be completed ahead of time. 我认为这项工作能提前完成.

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