動詞do在句中的作用可以概述為四句十二字："做"實義，助動詞，替前文，強語氣。

　　作用一：實義do

　　do作實義動詞時，有do, does, did, done, doing五種形式，還有及物、不及物之分。如：

　　1. vt. "做；研究；整理；完成"。如：

　　①The old man does an hour of sport every day.

　　②She did her homework at home last night.

　　③Mother was doing the cooking when I reached home.

　　④Have you done the exercises yet?

　　2. vi. "行動；工作；進展；足夠"。如：

　　①Kate does very well in her Chinese.

　　②How do you do?

　　③Well done!

　　④That will do.

　　作用二：助動do

　　do作助動詞時，只有do, does, did三種形式，無詞義，限用于含行為動詞的一般現在時和一般過去時兩種時態的否定句和疑問句中。如：

　　①They don't have any tickets for tonight's concert.

　　②Kate stayed at home last night, didn't she?

　　③How many books does the library have?