This verb is so basic that it s hard to find a good and complete explanation of the use of to do in most grammar books, although it s very difficult for nonnative speakers to divine when it should and should not be used. The verb to do has two distinct jobs. To do also has a second job description, a so-called auxiliary verb.
Children learn to do along with to be and to have as helping verbs.
In contexts where meaning is clear, especially in an answer to a question, auxiliary verbs can help avoid repetition. They stand in the place of a verb, holding the spot as though the main verb were still there. The auxiliary verb implies the presence of another verb, which is an ellipsis, or omission of words that are grammatically necessary but presumed to be understood. It s not my fault that he . . . They do not run their business the way they should.