There is / there are

We use there is and there are to say that something exists.
We use "there is" for singular and "there are" for plural.

Affirmative:
There is a student in the classroom.
There are four students in the classroom.

Negative: (doesn't exist)
There isn't a student in the classroom.
There aren't four students in the classroom.

Interrogative:
Is there a student in the classroom?
Are there four students in the classroom?

Short answers:
Yes, there is. Yes, there are. Or, No there isn't. No, there aren't.