RELATIVE CLAUSES: THE SENTENCES WHICH/THAT GIVE INFORMATION

Here's some basic and simple information on Relative Clauses.

Link to complete info  PERFECT GRAMMAR.

- WHAT'S A RELATIVE CLAUSE?
We use a Relative Clause to join two sentences in English or to give extra 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 RELATIVE CLAUSES.
Give us the necessary information to know what we're talking about.

I like the woman who lives next door. (If we don't say "who lives next door", we don't know which woman it is).

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.

 (clause after the subject of the sentence):

The people who / that live on the island are very friendly.

Link: EXERCISE OF RELATIVE PRONOUN AS SUBJECT.

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. 

(clause after the subject) The bike (which / that) I loved was stolen.

Link: EXERCISE OF RELATIVE PRONOUN AS OBJECT.

Link: EXERCISE OF MIXED DEFINING RELATIVES SUBJECT & OBJECT.

- NON DEFINING RELATIVE CLAUSES.

Give us extra information about something, but we don't need it to understand it.

I live in London, which has some fantastic parks. (Everybody knows where London is, "which has fantastic parks" is extra information).

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. A non-defining relative always goes between commas and a pause is made in speech.

(clause comes after the subject)

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.

(clause comes after the object)

Yesterday I called our friend Julie, who lives in New York.

Last week I bought a new computer, which I don't like now.

- WHOSE.

‘Whose’ is always the subject of the relative clause and can’t be left out. It replaces a possessive. It can be used for people and things.

The woman is coming tonight. Her car is a BMW.
→ The woman whose car is a BMW is coming tonight.

- WHEN / WHERE / WHY.

We can sometimes use these question words instead of relative pronouns and prepositions.

I live in a city. I study in the city.
→ I live in the city where I study.
→ I live in the city that / which I study in.
→ I live in the city in which I study.

- PREPOSITIONS IN RELATIVE CLAUSES.

If the verb in the relative clause needs a preposition, we put it at the end of the clause:

My brother met a woman. I used to work with the woman.
→ My brother met a woman (who / that) I used to work with.

If you want to hear an oral explanation on Relative Clauses, here's a video:
Video Relative Clauses explanation: