Clauses are parts of a sentence. There are two kinds of clause in English: independent and dependant.
Independent clauses contain a subject and a verb and so they are complete sentences. Another term for independent clause is main clause.
We left the house at 5 a.m.
Hamed eats an apple every day.
Dependent clauses are not complete sentences and require an independent clause in order to make sense. If you take the dependent clause out of a sentence, the speaker’s intended message is not expressed.
The car, which was brand new, needed a new wing mirror.
You went with her to the dentist, didn't you?
If you leave now, you'll miss the main event.
Relative clauses are dependent clauses that modify nouns. These clauses start with the relative pronouns "who", "whose", "which", "where", "when" or "that".
The car that I drove here in is actually my mother's.
His friend, who was wearing a bright red coat, only stayed for half an hour.
Gerald’s new job, which is at LingQ, has made him very happy.
Restrictive or Nonrestrictive?
Relative clauses can be either restrictive or nonrestrictive.
Restrictive clauses are essential to a sentence. They modify the noun or noun phrase and if they are removed, the sentence won't make sense or the message the speaker is trying to get across will be lost.
The muffins that I baked are all low sugar.
The hotel where Angela works will close down in a month.
The person who made us all late should have to clean the bus.
Nonrestrictive clauses are not essential to a sentence, they add extra information. If nonrestrictive clauses are removed the sentence still makes sense, the speaker’s message is fully expressed.
The little girl, who was carrying a yellow backpack, was waiting a long time for her dad.
The old church, which has needed a new roof for years, is very cold in the winter.
Fion Jones, who lives down the road, is going to walk the dog tomorrow morning.