An adjective clause is a dependent clause that functions as an adjective in a sentence. It provides additional information about a noun or pronoun in the main clause. Adjective clauses usually begin with a relative pronoun (such as who, whom, whose, which, or that) or a relative adverb (such as when, where, or why).

For example:

1. The book that I borrowed from the library is very interesting.
   • In this sentence, “that I borrowed from the library” is the adjective clause that describes the noun “book.”
2. The girl who won the competition is my best friend.
   • In this sentence, “who won the competition” is the adjective clause that describes the noun “girl.”
3. This is the house where I grew up.
   • In this sentence, “where I grew up” is the adjective clause that describes the noun “house.”
4. The reason why he left the party early is still unknown.
   • In this sentence, “why he left the party early” is the adjective clause that describes the noun “reason.”