Auxiliary verb is a verb that gives further semantic or syntactic information about a main or full verb

Do: A modal representing real actions

Do also plays the role of modal auxiliary verb for sentences referring to completed (simple) actions. The basic job of a modal is to make actions into explanatory phrases. Unlike the other modal auxiliaries (have or be), "Do" is not used in affirmative statements because unlike in perfect or continuous, in the case of simple, the action actually does happen.

Continuous: He is driving the car (Because he is only in the process of this action, the verb here serves as a descriptive adjective}

Perfect: He has driven the car (Because the action took place earlier, the verb here only serves as an explanation for some aspect of the current situation. For example, this is why there is no gas in the tank.)

Simple: He drives a car (he completes this action every day- it is a real action (without a modal) because something did in fact occur during the moment being spoken of.)

Do is used in questions and negations however because in those cases, there is no real action, but rather only the "idea" of the action.

Statement: He drives the car (There is an action)

Question: Does he drive the car? (No action)

Negation: He does not drive the car (No action)

Emphasis

The auxiliaries do, does, and did are also used for emphasis in positive declarative statements in which the verb otherwise contains only one word: "I do like this shirt!", "He does like this shirt", "I did like that shirt".

Properties of the English auxiliary verb

See also: English verbs

See also: List of English auxiliary verbs

Negation

Auxiliaries take not (or n't) to form the negative, e.g. cannot (can’t), will not (won’t), should not (shouldn’t), etc. In certain tenses, in questions, when a contracted auxiliary verb can be used, the position of the negative particle n't moves from the main verb to the auxiliary: cf. Does it not work? and Doesn't it work?.