In the simple present tense, do will function as an auxiliary to express the negative and to ask questions. (Does, however, is substituted for third-person, singular subjects in the present tense. The past tense did works with all persons, singular and plural.)
I don't study at night.
She doesn't work here anymore.
Do you attend this school?
Does he work here?
These verbs also work as "short answers," with the main verb omitted.
Does she work here? No, she doesn't work here.
With "yes-no" questions, the form of do goes in front of the subject and the main verb comes after the subject:
Did your grandmother know Truman?
Do wildflowers grow in your back yard?
Forms of do are useful in expressing similarity and differences in conjunction with so and neither.
My wife hates spinach and so does my son.
My wife doesn't like spinach; neither do I.
Do is also helpful because it means you don't have to repeat the verb:
Larry excelled in language studies; so did his brother.
Raoul studies as hard as his sister does.
The so-called emphatic do has many uses in English.
To add emphasis to an entire sentence: "He does like spinach. He really does!"
To add emphasis to an imperative: "Do come in." (actually softens the command)
To add emphasis to a frequency adverb: "He never did understand his father." "She always does manage to hurt her mother's feelings."
To contradict a negative statement: "You didn't do your homework, did you?" "Oh, but I did finish it."
To ask a clarifying question about a previous negative statement: "Ridwell didn't take the tools." "Then who did take the tools?"
To indicate a strong concession: "Although the Clintons denied any wrong-doing, they did return some of the gifts."
In the absence of other modal auxiliaries, a form of do is used in question and negative constructions known as the get passive:
Did Rinaldo get selected by the committee?
The audience didn't get riled up by the politician.