What are relative clauses and why should we take interest in them? To put simply these are those parts of sentences which:
identify which person or thing is being talked about (defining relative clauses) or simply give us extra information about the person or thing which has already been identified. (non-defining relative clauses) In writing non-defining clauses are separated from the rest of the sentence by commas. This is because they are not essential to the overall meaning.

Commas are not used in defining clauses. This is because the noun would be incomplete without the clause and would either be meaningless or have a different meaning.

Examples:

People who talk too loudly get on my nerves.

This is a defining relative clause. It tells you which people get on my nerves. If you take out the relative clause, the sentence reads
People get on my nerves.
which is a strange sentence because it is unlikely to be true.

(w języku polskim zawsze wstawiamy przecinek przed takimi wyrazami, jak: który, jaki itd. W wypadku zdań angielskich o zastosowaniu przecinka nie decyduje dany wyraz – e.g. who or which lecz to, czy mamy do czynienia ze zdaniem typu określającego [defining relative clause], czy raczej nie-określającego [non-defining].

A zatem podkreślmy; w zdaniach typu non-defining postawienie przecinka przed danym zwrotem oznacza, iż dana myśl, opis lub komentarz stanowią zaledwie dodatek, np. ciekawostkę, z której ewentualnie moglibyśmy zrezygnować. W zdaniach typu defining nie wolno nam wstawiać przecinków przed wyrazami takimi, jak who, which or what. Wypaczyłoby to sens danej wypowiedzi!).

consider some examples:

George W. Bush, who is the President of the USA, has to make the final decision whether or not to continue attacking Afganistan.

This is a non-defining relative clause. We already know who George Bush is so there is no need to identify him. The relative clause simply adds extra information.

Polish students who do not differentiate between these two types of clauses quite often try putting a comma each time they write who, that, which and whose. This is obviously the influence of the Polish language. In English on one occasion you will find a comma after who whereas on another occasion in a similar sentence there can be no comma at all.

Compare:

1). My sister who lives in Otwock is childless.

2). My sister, who lives in Otwock, is childless.

The first sentence suggest I have more than one sister and I am talking about the one living in Otwock (NOT the one from Warsaw). The second sentence implies that I have a sister and she has no children. The fact that she lives in Otwock is extra information I could well not mention.

Consider more examples:

1. My mother, who is 53, lives nearby.
2. William Golding, who won the Nobel Prize for Literature in 1983, died at the age of 81.
3. Young people who leave full-time education at the age of 16 often come to regret it later.
4. Children whose parents smoke will often become smokers themselves.
5. When I lived in Oxford, which is some time ago, we used to go to Stratford quite often at the weekend.
6. George, who was standing by the door, called over to me.
7. That woman who phoned last night said she’d phone again this morning.
8. The cheque you said you had posted still hasn’t arrived.
9. People who live in glass houses shouldn’t throw stones.
10. He studied at The Eckersley School of English, which is right in the centre of Oxford.
11. Jack is the boy outside who is playing tennis with Ted.

12. Sam is the doctor who treated the Governor after the accident.

13. Timothy, who is writing a novel at this moment, is a film director.

14. Sarah is the woman who was elected to the town council last year.

15. Keith, who lives next to me, makes a lot of noise every morning.

16. Sharon is the secretary who is preparing the reports for the boss.

17. The driver, who was warned by the police twice, finally got a traffic ticket.

18. My two best friends who are studying in America are quite homesick

19. The new tax laws ,which Harry has studied in depth, are quite complicated.

20. My new car , which is crossing the Atlantic on a boat at this moment, was bought in the USA.

Sometimes a relative clause refers to the whole sentence before it.
My father didn’t let me go to the disco, which upset me.
What upset me was not the disco but the fact my father didn’t let me go there.

My father didn’t let me go to the disco which was in the city centre.
Here the bolt type part of the sentence does NOT refer to the whole sentence but ONLY describes the disco!

The most common relative pronouns are:
Who, which, that, whose, when, where, why
The person who did this will have to report to the headmaster.
The girl, whose mother you know all too well, promised to come to the party.
December is the month when we will be sitting the FC exams.
Dzierżoniow, where I was born, is situated in the south-east part of Poland.
The reason why I did not tell you the truth is that I love you.
In defining relative clauses who or which can be replaced by that.
The man who is looking at me is a policeman in disguise.
The man that is looking at me is a policeman in disguise.
In non-defining relative clauses that cannot substitute who!
The relative pronoun can be left out if it is the object of the verb in the relative clause.
The man that I fell in love with is older than my own father.
The man who I fell in love with is older than my own father.
who – I fell in love with him
The man I fell in love with is older than my own father.
Is it difficult to use relative clauses well, then?
With a little bit of practise it does not have to be so. See for yourself.