There are two kinds of adjective clauses in English: defining and non-defining. This lesson explains defining adjective clauses.
What's a defining adjective clause?
First, some examples:
|I know a guy who has five cats.||who has five cats||It describes “guy”|
|The book I bought cost $30.||I bought||It describes “book”|
Please notice how the clause defines the noun it describes — it tells us which one we're talking about, and without it, we don't know which one we're talking about. If I say: “I know a guy,” you don't know which guy I'm talking about, but if I say “I know a guy who has five cats,” you know which guy I'm talking about — he's the one with five cats. That's why we call this type of adjective clause “defining” — it defines the noun; it tells us which one we're talking about.