We can use relative clauses to join two English sentences, or to give more information about something. I bought a new car. It is very fast. → I bought a new car that is very fast. She lives in New York. She likes living in New York. → She lives in New York, which she likes. Defining and Non-defining A defining relative clause tells which noun we are talking about:

like the woman who lives next door. (If I don't say 'who lives next door', then we don't know which woman I mean). A non-defining relative clause gives us extra information about something. We don't need this information to understand the sentence.
I live in London, which has some fantastic parks. (Everybody knows where London is, so 'which has some fantastic parks' is extra information). DEFINING RELATIVE CLAUSES: 1: The relative pronoun is the subject : First, let's consider when the relative pronoun is the subject of a defining relative clause. We can use 'who', 'which' or 'that'. We use 'who' for people and 'which' for things. We can use 'that' for people or things. The relative clause can come after the subject or the object of the sentence. We can't drop the relative pronoun. For example (clause after the object of the sentence):
I'm looking for a secretary who / that can use a computer well.
She has a son who / that is a doctor.
We bought a house which / that is 200 years old.
sent a letter which / that arrived three weeks later. More examples (clause after the subject of the sentence):
The people who / that live on the island are very friendly.
The man who / that phoned is my brother.
The camera which / that costs £100 is over there.
The house which / that belongs to Julie is in London.

2: The relative pronoun is the object : Next, let's talk about when the relative pronoun is the object of the clause. In this case we can drop the relative pronoun if we want to. Again, the clause can come after the subject or the object of the sentence. Here are some examples: (Clause after the object)

She loves the chocolate (which / that) I bought.
We went to the village (which / that) Lucy recommended.
John met a woman (who / that) I had been to school with.
The police arrested a man (who / that) Jill worked with. (Clause after the subject)
The bike (which / that) I loved was stolen.
The university (which / that) she likes is famous.
The woman (who / that) my brother loves is from Mexico.
The doctor (who / that) my grandmother liked lives in New York. NON-DEFINING RELATIVE CLAUSES: We don't use 'that' in non-defining relative clauses, so we need to use 'which' if the pronoun refers to a thing, and 'who' if it refers to a person. We can't drop the relative pronoun in this kind of clause, even if the relative pronoun is the subject of the clause. (Clause comes after the subject)
My boss, who is very nice, lives in Manchester.
My sister, who I live with, knows a lot about cars.
My bicycle, which I've had for more than ten years, is falling apart.
My mother's house, which I grew up in, is very small. (Clause comes after the object)
Yesterday I called our friend Julie, who lives in New York.
The photographer called to the Queen, who looked annoyed.
Last week I bought a new computer, which I don't like now.
I really love the new Chinese restaurant, which we went to last night. PREPOSITIONS AND RELATIVE CLAUSES If the verb in the relative clause needs a preposition, we put it at the end of the clause: For example: listen to The music is good. Julie listens to the music. → The music (which / that) Julie listens to is good. work with My brother met a woman. I used to work with the woman. → My brother met a woman (who / that) I used to work with. go to The country is very hot. He went to the country. → The country (which / that) he went to is very hot.