The formal subject there is used to indicate that something is somewhere:

There were some paparazzi pictures in the magazine that were beyond intrusive!

When something is somewhere, use the structure there is or there are. When a noun is singular, the verb be is in the present form is. When a noun is plural, the verb be is in the present form are:

There is a cat in my room.

There are three bedrooms in this house.

When something is not somewhere, use the structure there isn’t or there aren’t:

There isn’t a safe in this room.

Note that the structure there is/there are is conjugated according to the tense:

There was a cat in my room.

The formal subject it is always followed by the verb be which is always in singular and conjugated according to the tense. It is used to express time, distance, opinion, weather and to highlight a particular component part of a sentence among other things:

It‘s absolutely wrong to plagiarise someone else’s work.

It was a hot summer day.