Когда употребляется предлог are а когда is?

Are and is are the forms of the auxiliary verb to be. Are - is used with plural, and is - is used with singular.

You are in the park.

He is a student.

She is an actress.

We are drinking tea with milk now.

They are in the room.

Mary is reading a book.

Tom is writing a letter.

Mary and Tom are playing basketball.

They are swimming in the river.

Is she drinking coffee?

We are watching an interesting film.

The film is very interesting.