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常见的助动词有：do、be、have，它们的用法各不相同，常见用法如下：
一、be的用法
1.be+动词不定式to do。
①表示最近、未来的计划或者安排。
Eg. He is to go to England next week.
②表示命令。
Eg. You are to explain this.
③表示征求意见。
Who is to go there?
④表示约定。
Eg. We are to meet at the school gate.
2.be+现在分词，构成进行时态。
Eg. They are having a class.
3.be+过去分词，构成被动语态。
The window was broken by Mary.
二、do的用法
1.构成一般疑问句。
Eg. Do you want to pass the exam?
2.do + not 构成否定句。
Eg. He doesn't like to study.
3.构成否定祈使句。
Eg. Don't go there.
4.放在动词原形前，用来加强该动词的语气。
Eg. I do miss you. 我真的想你。
5.用于倒装句。
Eg. Never did I hear of such a thing.
注意： 引导此类倒装句的副词有never、seldom、rarely、little、only、 so、well等。
三、have的用法
1.have +过去分词,表示完成。
Eg. By the end of last month， they had finished half of their work.
2.have + been +现在分词，构成完成进行时，即现在完成时。
Eg. I have been studying English for ten years.
3.have+been +过去分词，构成完成时态的被动语态。
Eg. English has been taught in China for many years.
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