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.)