Adjective Clauses

Adjective clauses, also known as relative clauses, modify nouns, just as an adjective would do.
They are always dependent clauses; for this reason, they always form complex sentences. 
The subject of the adjective clause is the relative pronoun

• that, which, who, whom, whose, when, where

You use that or which for things in subject position
You use that or who for people in subject position
You use whom for people in object position
You use whose for people in a possessive relationship
You use where for places
You use when for time

Example
The doctor that operated on my sister is standing next to the desk. 
Independent clause: The doctor is standing next to the desk. 
Dependent Clause: that operated on my sister
Notice that the function of the adjective clause is to tell us "what doctor"

More Examples

• The tree that is in front of my house is an oak. 
• They saw the girl whom they mugged last week. 
• I called Mike Thompson, whose sister used to be my girlfriend.
• They demolished the house where my mother grew up. 
• I'll always remember the day when I flew for the first time.