Do has three main uses.
As an Auxiliary Verb
The auxiliary do is used to make emphatic, interrogative and negative verb forms. It is followed by an infinitive without to.
- Did you post the letters?
- Do you like football?
- This doesn’t taste very nice
- Do sit down.
- I do admit that I was wrong.
- He did come.
Note that we use do to make questions and negatives with ordinary verbs, but not with other auxiliary verbs.
- Do you like dancing? (NOT Like you dancing?)
- I don’t like reading. (NOT I like not reading.)
- Are they sleeping? (NOT Do they are sleeping?)
- I will not come. (NOT I do not will come.)
- Will you help me? (NOT Do you will help me?)
- I can't see anything. (NOT I do not can see anything.)
To make imperative sentences
Do can be used with be to make imperative sentences.
- Don't be silly!
- Do be quite!
- Do be a good child.
As an ordinary verb
Do is also an ordinary verb. The ordinary verb do can refer to almost any kind of activity.
- What were you doing in the morning?
- What did you do then?
- Do as I tell you.
- It was a stupid thing to do.
- Can't you do it yourself?
- You are a grown up man now. You should be able to do things on your own.
- Do with me what you like.
- I don't know what I did to make her angry.
The auxiliary do and ordinary do can sometimes occur together.
- What did you do then? (Did - auxiliary, do- ordinary)
- I don't do well in mathematics. (Don't-auxiliary, do- ordinary)
Note that the ordinary do has infinitives (to do, to be done) and participles (doing, done).
Do - Other uses
Do can be used to avoid repeating a verb or a verb phrase.
- She looks much younger than her husband does. (= …her husband looks.)
- May I join you? Please do. (= Please join us.)
- Who said that? I did. (= I said that.)
- I thought I would take a day off school today. No you don't. (= You are not going to take a day off.)