ADJECTIVE CLAUSE

Definition :
An adjective clause is a clause that describes a word or a group of words in another clause. Adjective clause are often part of a complex sentence.
  
Adjective clauses use 'that', 'who', 'whom', and 'which' to begin the clause :
- That used for humans, animal, and things, in subject or object position.
- Who used for humans in subject position.
- Whom  used for humans in object position.
- Which used for things and animals in subject or object position.
- When used for preposition (in, on , at)
- Where used for place(s).
- Whose used to indicate ownership.      

Example :
> I will introduce you to a friend. He runs a successful business.
   I will introduce you to a friend who runs a successful business. 

>  I will introduce you to a friend.You have never met him before.
   I will introduce you to a friend who(m) you have never met him before.

> The book is about religion. I bought it in Gramedia bookstore last week. 
   The book which I bought it in Gramedia bookstore last week is about 
   religion.

> The song was very popular in 1990's. I am listening to it.
   The song that I am listening to was very popular in 1990's. 

> I will take you to the restaurant. I usually have lunch there.
   I will take you to the restaurant where I usually have lunch.

>  we will never forget the day. My wife gave birth to my son then.
    we will never forget the day when my wife gave birth to my son.

> The old lady has a painting. its value is inestimable.
     The old lady has a painting whose value is inestimable.