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.时，意为“做；研究；整理；完成”。如：

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

② She did her homework at home last night.

③ Mother was doing the cooking when I reached home.

④ Have you done the exercises yet?

2. vi.时，意为“行动；工作；进展；足够”。如：

① Kate does very well in her Chinese.

② How do you do?

③ Well done!

④ That 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：

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

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

③ How many books does the library have?

④ She 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：

① Tom runs much faster than you do.

②— Lucy, can you get some more tea, please?

— Sure. I'll do it right away.

③— Who broke the cup?

— Mimi did.

④— 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：

① Do be careful.

② Don't tell a lie.

③ He did come.

④— You often go to the park.

— So we do.