English sentences require a subject:
The doctor (= subject) owned an expensive car.
Mary (= subject) worked at a restaurant.
Exception
The imperative (giving orders) does not require a subject:
Go away!
Stop smoking!
If a sentence does not have any other subject, we can use there or it as a subject (called a dummy subject).
If there is no other subject, we use it to talk about:
Times and dates
It’s nearly five o’clock.
The weather
It’s snowing.
Distances
It’s three miles to school.
There are more uses of it, but these are some of the most common ones.
If there is no other subject, we use there to talk about:
Where or when something is
There is an interesting book on the shelf.
A number or amount
There were ten people at the meeting.
Something existing or happening
There’s a small problem.
There was a nasty fight.
There are more uses of there, but these are some of the most common ones.