We have already seen that adjective clauses can be
classified into subject-pattern clauses, object-pattern
clauses, possessive clauses, and clauses with where
and when. They can also be classified in another way:
into restrictive and non-restrictive clauses.

Restrictive Clauses

An adjective clause is restrictive if it is needed to
identify (specify, clarify) which noun phrase is being
talked about in the sentence that contains the clause.
If the clause is restrictive, it it will answer the question
"Which (noun) _____ ?"

Examples

A: Some person called Judy last night.

B: Which person called Judy last night?

A: The person (who[m] / that) she met
at Ken's party.

This clause is restrictive because it's needed
to identify "the person."

_________________________________________

A: Some person called again this afternoon.

B: Which person called again this afternoon?

A: The person who (that) called yesterday.

This clause is also restrictive; again, it's needed
to specify "the person."

_________________________________________

A: The soup is too salty.

B: Which soup?

A: The soup (that / which) Sally made.

This clause is restrictive, too: it shows which soup
is being referred to.

_________________________________________

A: One of those cars belongs to Tina.

B: Which one?

A: The one that's (which is) parked beside Joe's.

Once again, the clause is restrictive; it's necessary
information if A wants B to know which car she is
referring to.

__________________________________________

Non-Restrictive Clauses

An adjective clause is non-restrictive if the clause
is not needed to identify which noun phrase is being
talked about. This does not mean that the clause
should be omitted from the sentence. Instead, it
means that the clause gives important information,
but the information is extra.

Examples

This clause is non-restrictive. It's not needed
to identify the person who called Judy because
the person is identified by name: Jim Peterson.

_________________________________________

This clause is also non-restrictive. It's not needed
to show which person called again this afternoon;
your mother makes the person's identity very clear.

_________________________________________

The clause inside the commas is not needed to show
which soup is being referred to because the modifiers
Sally's, vegetable, and beef make this very clear.
Knowing that this soup is in the big white bowl is extra
information, so the clause "which is in the big white
bowl" is non-restrictive.

_________________________________________

The clause inside the commas is also non-restrictive.
It's not needed to to show which car is being referred to
because the modifiers Tina's and new make this very
clear. For this reason, the clause "which she bought last
week" is extra information,