An adjective clause is a clause that functions as an adjective. If that seems circular, some examples will make it clearer.
First, a refresher on what an adjective does:
I love looking at the blue sky.
The adjective blue describes (or modifies) the noun sky.
Now let’s see an example of a clause doing the same thing:
The store where I used to work just went out of business.
The clause where I used to work functions as an adjective to modify the noun store. It tells us which store the writer is referring to.
Adjective clauses are introduced by a relative pronoun or a subordinating conjunction.
Relative pronouns include that, which, whichever, who, whom, and whose, among others.
Subordinating conjunctions are words like when, where, why, how, and whatever.
Two Types of Adjective Clauses
Adjective clauses break into two categories: restrictive clauses and nonrestrictive clauses.
Restrictive Clauses
A restrictive clause modifies a noun with a narrowing or limiting description, giving information that is essential to understanding the modified noun (restrictive clauses are sometimes called essential clauses).
Example
My sister who lives in Utah sent me flowers.
The sentence above assumes the writer has more than one sister. The clause who lives in Utah tells us which of the sisters the writer is referring to.
More Examples of Restrictive Clauses
(The noun is in red, the adjective clause is in blue, and the relative pronoun or subordinating conjunction is in bold blue)
The police searched the car that the thief used to get away.
I am the one who runs this show.
Peter sent prayers to all the people whose homes were lost in the fire.
The day when all people can come together is a day worth wishing for.
I want to live in a house where prices are low.
In each case, the restrictive clause (blue) specifies or narrows the meaning of the noun (red)
Nonrestrictive Clauses
A nonrestrictive clause modifies a noun by adding information. This additional information is nonessential, which means that removing the information would not change the meaning of the noun. Nonrestrictive adjective clauses are set off by commas.
Example
My sister, who lives in Utah, sent me flowers.
Here, the writer refers to only one sister (We assume the writer has only one sister or that the audience already knows which sister is being referred to). Again, we know the adjective clause is nonessential because we can remove it without changing the meaning:
My sister sent me flower.
More examples of Nonrestrictive Clauses
The man, whom police had interviewed, disappeared into the night.
Whatever he does, He is sure to succeed.
The book, which arrived late, was more damaged than the seller indicated.