We use relative clauses to describe or tell something more about a person or a thing. A relative clause always starts with a relative pronoun. Relative pronouns in English are who, which, whose and that. Whom is also used referring to an object or following a preposition, but it's very formal.
Who - Whose - Whom - Which - That
Who – when we talk about people
Which – when we talk about things or animals
Whose – when we talk about possessions.
That – when we talk about people, things or animals
Whom – when we talk about people (object) or with a preposition – very formal
I dated a girl. She lives next door.
I dated a girl who lives next door.
I dated a girl that lives next door.
This is my new laptop. I bought it two days ago.
This is my new laptop which I bought two days ago.
This is my new laptop that I bought two days ago.
I was invited by Peter. I met him last Monday.
I was invited by Peter whom I met last Monday.
I was invited by Peter who I met last Monday.
To whom it may concern.
For whom did you bake this apple pie?
I have a friend. His brother is a mechanic.
I have a friend whose brother is a mechanic.
Whose bike is it? It’s Susan’s.
There are also non-defining relative clauses. We use them to give extra information about the person or thing that is not important. We use commas in non-defining relative clauses, but we don’t use the relative pronoun "that".
Sarah, who I met yesterday, worked at my father’s company.
Do you know that woman, who is standing at the corner?
My motorbike, which is very old, broke down last weekend.
The relative pronoun can be left out when the pronoun refers to the object of a sentence.
This is the watch her parents bought her for her birthday.
The girls we met yesterday are very smart.