"Be, do, have" are the main auxiliary verbs in English. We need them to make negatives and questions, but they have a lot more uses
Examples
DIANE: Well, what do you think of the movie?
MIRANDA: I guess it was good, but I didn't like the ending.
DIANE: Neither did I. It felt too dramatic, didn't it?
MIRANDA: Yes, it did. I'd like to get a more realistic interpretation.
DIANE: Do you know that the movie has been shortlisted for the Academy Award?
MIRANDA: Has it? George Lucas does know how to make award-winning films.
DIANE: Right, he does.
We use "so" and "neither" to agree to a negative statement.
— You know I didn't like the movie.
— Neither did I.