Here are two rules to follow when using defining adjective clauses.
There are some simple rules which can help you to make good defining adjective clauses and avoid mistakes.
1. Never use commas with a defining adjective clauses.
Right: I like people who are on time. Wrong: I like people, who are on time.
2. Relative pronouns: who, which, that, whose, where
A relative pronoun is a word which signals the beginning of an adjective clause. If the adjective clause describes a person, we can use 'who' or 'that'.
The family who live next door to us bought a new car.
The family that live next door to us bought a new car.
If the adjective clause describes a thing, we can use 'which' or 'that'.
The car that they bought was red.
The car which they bought was red.
If the adjective clause shows some kind of possession or who something belongs to, then we use 'whose'.
We threw away the chairs whose legs were broken.
She's the girl whose boyfriend was in an accident.
Please notice we can use 'whose' for people or things.
We can use the relative pronoun 'where' for places sometimes.
Do you know a place where I can buy an interesting birthday card?
Finally, the tricky rule. We can omit the relative pronoun if it isn't the subject of the clause it's in. However, if the relative pronoun is the subject of the clause it's in, then we have to use it — we can't throw it away. For example:
The shirt that she gave me was too big.
The shirt she gave me was too big.
In the first example, please notice the defining adjective clause is 'that she gave me'. In the second example, notice how it's possible to drop the relative pronoun because it's not the subject of the verb in the clause. (The subject of the verb in the clause is 'she'.) Both examples are good English — you can use either one.
Okay, a new example.
Right: The police arrested the guy who hit my car. Wrong: The police arrested the guy hit my car.
The second example is bad English because “who” is the subject of “hit”. The verb in the adjective clause needs a subject, and that subject is the relative pronoun “who”, so we can't omit it.