助动词 共有 do ?have ?be ?will ?shall 等五个。助动词用来 构成疑问句，各种时态，强调等。
Do you do homework every day? 构成一般疑问句（第一个 do）
I didn't go to school yesterday. 构成否定用
I have studied English for ten years. 构成 现在完成时态
I am learning English now. 构成 现在进行时态
I will go to school tomorrow. 构成 一般将来时态。
I did finish my homework. ?构成强调（did）
没有动词来修饰主语的时候可以用助动词，当主语有自己的动词就不需要。
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