動詞do在句中的作用可以概述爲四句十二字：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，限用於含行爲動詞的.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?
④She doesn't do the washing in the evening.