Adjective clauses (or relative clauses) are a type of subordinate clause that act as adjectives.
The whole clause does the job of an adjective.
Take a look at this sentence:
The happy woman danced across the street.
Happy is an adjective modifying the noun woman.
It is telling us which woman.
Do you remember that which one is one of the adjective questions?
Which woman? The happy woman.
Now, look at this sentence.
The woman who looks happy danced across the street.
This time, a whole clause is modifying the noun woman!
The clause is still telling us which woman.
Which woman? The woman who looks happy.
This clause is an adjective clause. It is a group of words with a subject and a verb, and it is acting as one part of speech - an adjective.
Most of the time, relative clauses are introduced by relative pronouns.
who, whom, whose, that, which
Here are examples using these relative pronouns:
Relative pronouns link the clause with the word that the clause is modifying.
Did you notice that the noun that comes directly before the clause is the noun that the clause is modifying?
Good. I'm glad that you saw that. :)
The cool thing about relative pronouns is that they also act as the subject, object, or some kind of modifier within the adjective clause.
Let's use this sentence as an example.
This is the house that Jack built.
The independent clause is This is the house. The relative clause is that Jack built. Notice that both clauses have a subject and a verb.
That is introducing the relative clause. It is linking the word house with the whole clause.
That is also acting as the direct object within the clause.
Jack = subject built = verb that = direct object
Look at the sentence diagram, and it will help you see what I mean.
You'll find more information and sentence diagrams on the relative pronouns page.
Sometimes the relative pronoun is missing from the relative clause.
You can still have an adjective clause without the relative pronoun.
Here is an example of a sentence with the relative pronoun that.
The adjective clause is that I love.
Can you think of how you could express that same idea without the relative pronoun?
Now the adjective clause is just the words I love, but the word that is implied. It's as if the sentence says:
Whenever words are implied in a sentence, it is called an elliptical.
The technical name for a missing relative pronoun is an elliptical relative pronoun.
If you want to diagram the sentence, diagram the implied relative pronoun in parenthesis.
While most of the time relative clauses are introduced by relative pronouns, sometimes they are introduced by relative adverbs (where, when, why).
Here are some examples. Notice that the clause is still modifying a noun.