As a semantic and auxiliary verb in the same sentence.
What did you do with my book?
I didn't do anything.
Note that this is only possible in questions and negations.
It acts as an emotional intensifier.
Mom,I did pass my test. Ask my teacher. (Mom apparently didn't believe it at first, so the child had to add expression)
Are you kidding? I do see nothing in the sky.
A difficult case is the use of both does and does with collective nouns such as: group, family, firm, company, class, crowd, etc.
My family do everything together. (This means each member of the family, several people, so it is spelled do).
Government does people for with its indifference. (The government here is a collective image, a whole, so does).
In the American version, it is preferable to use the singular with such words. And the corresponding third person verb form.
For a brief answer to the question.
- Do you prefer apples?
- I do.
- I don't.
The answers make sense: Yes, I prefer apples to other fruits. Or vice versa: I don't like them. You don't have to answer in full sentences. Short means "more natural."
Forms separating questions, "with a tail". Tails translates to "isn't it", "isn't it"?
Alex watches movies at the cinema, doesn't he?
My brothers don't like to read books, do they?
Attention, attention: if the first part of the sentence is positive, the tail is negative and vice versa.