In English language there are 7 types of clauses, relative clauses are amongst them. (relative clause = relatiivlause e. kõrvallause, mis enamasti laiendab pealause nimisõnafraasi.)
A relative clause gives additional information about a noun, a pronoun or a verb. They contain relative pronouns:
who (whom) = PEOPLE
which = THINGS
whose = POSSESSIONS
when = TIME
where = PLACE
what = VERBS
that = PEOPLE or THINGS (it's a relative adverb)
Words who / which /that can be omitted if they act as objects. Don't use commas in relative clauses, but you have to use them in non-relative / non-defining sentences. In formal language WHOM is often used instead of who or who to
Examples
1) This the man who helped me in the accident. (the man = subject, therefore the pronoun who can't be omitted. It is a relative clause => no commas needed. )
2) The car (that) I bought last year broke down. ( the car = object, therefore the pronoun that can be omitted.)
3) The house, whose roof was damaged, is being repaired. (the house = subject, therefore the pronoun can't be omitted. It's a non-defining relative clause => commas are needed.)
PS! subject - alus, object - sihitis
The easiest way to check whether a sentence is a non-defining relative clause => read the sentence without clause and a non-defining clause makes another independent sentence.
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How to change formal sentences into less formal? Which relative pronoun is more formal: which or that, who / that, whom / who to?
1) The car in which we drove to London belongs to my mum.
2) This is the address to which you should write.
3) The exam about which I'm worried is on Monday.
4) Chris is the friend with whom I went to Italy.
5) The man about whom I told you is over there.
6) Who is the girl to whom you were talking?
