Defining relative clauses

Have you read the message (that) I sent you yesterday?
This example contains information about the noun message. She was talking to some people (who) she works with.
This example contains information about the noun people. That's the hotel where we stayed on holiday.
This example contains information about the noun hotel.
Have you read the message (that) I sent you yesterday? The relative clause - that I sent you yesterday - tells us which message. Without the relative clause, the question - Have you read the message? - is unclear. You would ask Which message? That's the hotel we stayed in last year. The defining relative clause - we stayed in last year - tells us which hotel. Without the relative clause, the sentence - That's the hotel - makes little sense unless the hotel was was specified earlier.
A cat is a small animal that people often keep as a pet.
The relative clause - that people often keep as a pet - tells us what kind of animal. Without the relative clause, the sentence A cat is a small animal. is less clear. I'm looking for a computer that I can use when I am travelling. The relative clause - that I can use when I'm travelling - tells us what kind of computer you are looking for.

The relative clause usually comes immediately after the noun that it relates to. I read the book that you gave me on the train.
In this example, the relative clause - that you gave me - relates to the book, not the train. (You gave me a book, not a train.)

If we change the position of the relative clause, it sounds strange:
I read the book on the train that you gave me.
Here the relative clause - that you gave me - sounds as if it relates to the train. (You gave me a train!)

The relative pronoun in a defining relative clause is sometimes left out (omitted), especially in spoken English.
When the relative pronoun is not the subject of the relative clause, it can be omitted. That's the woman that I met at the conference. The relative pronoun that relates to the woman. The woman is the object of the relative clause in this sentence. I is the subject of the relative clause, so the relative pronoun that can be omitted. That's the woman I met at the conference.
*Note: Relative pronouns can never omitted in non-defining relative clauses.
When the relative pronoun is the subject of the relative clause, it must be written. That's the man who interviewed me. The relative pronoun who relates to the noun the man and is the subject of the relative clause. It cannot be omitted.
That's the man interviewed me is incorrect.

There's the bag that I lost.
Where's the bag which I gave you?
The woman that I met.
The woman who I met.
The man whose car was stolen.
The town where we lived.
The town which / that we lived in.
The town in which we lived.
The last of these is formal and more common in written English.
It's a day when everyone celebrates.
It's a day on which everyone celebrates.

 

It was a time when most people were very poor.
It was a time in which most people were very poor.
What is sometimes used in error as a relative pronoun. It means the thing(s) which, so it cannot be used after a noun in the same way as a relative pronoun. I want to see what they have in the sale.
I want to see the things which / that they have in the sale.
I agreed with what she said.
I agreed with the thing(s) that / which she said. Where's the thing what we use for cleaning the sink?
This should be the thing (which / that) we use for cleaning the sink.

The relative pronouns where and which are both used to talk about places. However, there are differences in the way that they are used.
That's the house where we live.
That's the house (which / that) we live in.
In normal (not very formal) English, it is common to put the preposition at the end of the sentence. That's the house in which we live.
In a more formal style, the preposition goes before the pronoun. Only which (not that) is used. A relative clause with where always contains the idea of a preposition.
In the example above, we are saying, That's a house. We live in that house. If you use which or that, the preposition must be included. That's the house which we live. This is incorrect.
New York is a city where has many tall buildings.
This is incorrect as where appears to be the subject of the clause. New York is a city which has many tall buildings.
This is correct. New York is a city where there are many tall buildings.
This is correct. There acts like a subject in this case.
When there is no preposition in the information contained in the relative clause, we use which, that, who etc.
That's the house (which / that) I bought.
That's the house (which / that) I share with my friends. In these examples, the pronoun (which or that) can be omitted as it is not the subject of the clause.

There are other ways of adding information to nouns. For example, by using adjectives or by using other nouns to make compound nouns. Sometimes this is more efficient when the information you want to add can be expressed in one or two words:

When the information is longer and more complex, a relative clause is often necessary:

We stayed in a hotel that had a beautiful view over Lake Geneva.

There is too much information here to express with adjectives or compound nouns. We
can't say, We stayed in a beautiful Lake Geneva view hotel.