An adjective clause is used to describe a noun. A relative pronoun is usually used to introduce an adjective clause:
Who: used for humans in subject position:
Hans, who is an architect, lives in Berlin.
Whom: used for humans in object position:
Marike, whom Hans knows well, is an interior decorator.
Which: used for things and animals in subject or object position:
Marike has a dog which follows her everywhere.
That: used for humans, animals and things, in subject or object position (but see below): Marike is decorating a house that Hans designed.
Whose: used for humans, animalsi or objects to give information about their possessions.
The girl whose dress is red is my best friend.
Where: used for places
The hotel where we stayed last summer was very beautiful.
When: used for time
My baby was born in the year when I moved to Italy.
give extra information about the noun, but they are not essential, they give extra information:
Ataturk , who was the greatest leader of all times , created a modern country from the ashes of Ottoman Empire.
(We know who Ataturk is, and the clause used here gives extra information about him.)
The desk in the corner , which is covered in books , is mine.
(We don’t need this information in order to understand the sentence. “The desk in the corner is mine” is a good sentence on its own — we still know which desk is referred to.)
Be careful!!! : non-defining clauses are usually separated by commas.
“that” is not usually used in this kind of context.
give essential(necessary) information about the noun:
The package that arrived this morning is on the desk.
(We need this information in order to understand the sentence. Without the relative clause, we don’t know which package is being referred to.
Be careful!!! “that” is often used in defining relative clauses, and they are not separated by commas.