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?

　　④She doesn't do the washing in the evening.

　　作用三：替代do

　　為避免動詞的重復，使語言簡練，常以do, does, did替代前文所用的行為動詞。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：語氣do

　　為突出感情色彩，do常用于祈使句、強調句和倒裝句中，以加強語氣。如：

　　①Do be careful.

　　②Don't tell a lie.

　　③He did come.

　　④-You often go to the park.

　　-So we do.