Do

Do can be used as a full verb or as an auxiliary verb. It is the third most frequently used verb in the English language.

Full verb
As a full verb, do has a variety of meanings (I must do something about that. Have you done your homework?).

Auxiliary verb
As an auxiliary verb, do is used for three forms of the full verb: the negative (She doesn't like you), the interrogative (Did Paul say anything to you?) and the emphatic (We did enjoy the play). It also stands in for the full verb in, for example, question tags (That went off quite well, didn't it?).

Difficulties
Some students may, at first, have problems getting to grips with things like What do you do at weekends?; What do you do when that happens? (repeated use of the same word); I do like a bit of gorgonzola! (affirmative use of an auxiliary) or humdingers like: What do you think I should do? (two auxiliaries).

Pointing out word order is particularly useful in sorting out any possible doubts in such cases.

Forms

 * As a full verb: do, does, did, done, doing;
 * As an auxiliary verb: do/don't, does/doesn't, did/didn't.