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.