动词do在句中的作用可以概述为四句十二字："做"实义，助动词，替前文，强语气。
作用一：实义do
do作实义动词时，有do, does, did, done, doing五种形式，还有及物、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. "行动;工作;进展;足够"。如：
①Kate does very well in her Chinese.
②How do you do?
③Well done!
④That will do.
作用二：助动do
do作助动词时，只有do, does, did三种形式，无词义，限用于含行为动词的一般现在时和一般过去时两种时态的否定句和疑问句中。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.
作用三：替代do
为避免动词的重复，使语言简练，常以do, does, did替代前文所用的行为动词。如：
①Tom runs much faster than you do.
②-Lucy, can you get some more tea, please?
-Sure. I'll do it right away.
②-Who broke the cup?
-Mimi did.
③-I like bananas.
-So does he.
作用四：语气do
为突出感情色彩，do常用于祈使句、强调句和倒装句中，以加强语气。如：
①Do be careful.
②Don't tell a lie.
③He did come.
④-You often go to the park.
-So we do.