We use there is (singular) / there are (plural)to say that sth is located in the place or exists:

e.g. There is a book on the desk. / There are manybooks on the shelf.

We use there isn’t (singular) / there aren’t (plural)to say that sth isn’t located in the place or doesn’t exist:

e.g. There isn’t a book on the desk. / There aren’t any books on the shelf.

We use Is there…? (singular) / Are there...? (plural)to ask whether sth is located in the place or exists:

e.g. Is there a book on the desk? / Are there any books on the shelf?