There is / There are

They are used to express the existance of something, just to show it.

"There is" is used with singular elements and "There are" with plural elements.

- There is a cat on the roof.

- There are cats on the roof.

They work as any sentence with the verb "to be", so they make direct negatives and subject-verb inversion in the interrogatives.

- There isn't a cat on the roof.

- There aren't cats on the roof.

- Is there a cat on the roof?

- Are there cats on the roof?