Theory:

Adjective or Relative Clauses

An ADJECTIVE CLAUSE, which is also called a RELATIVE CLAUSE, is a group of words which include a subject and a verb. It acts like an adjective and begins with a relative pronoun - WHO, WHOM, WHOSE, THAT, or WHICH, or a relative adverb - WHEN, WHERE, or WHY.

My brother, who lives in France, is a dentist. (adjective clause)

There are several different kinds of relative pronouns and adverbs depending on what is being described:

person - who

thing - which

place - where

time - when

whose - to show possession

That man, WHO lives in this house, is French.(relative clause)

My car, WHICH I sold yesterday, was red.(relative clause)

The cinema, WHERE we saw the film, was in London.(relative clause)

The year, WHEN we met, was 1991.(relative clause)

That man, WHOSE dog is black, is French.(relative clause)

There are two kinds of adjective clauses, DEFINING or NON DEFINING.

The sentence, My brother who lives in France is a dentist, can have two possible meanings.

If I only have one brother, the clause who lives in France gives some extra information that isn't essential for the meaning of the sentence.

Such a clause is called a NON-DEFINING relative clause.

If I have several brothers and I need the clause who lives in France to identify which brother I mean, then the clause is fundamental for the correct meaning of the sentence, and the clause is called a DEFINING CLAUSE.

A defining clause does not need commas, and a non defining relative clause is always separated by commas. So:

My brother, who lives in France, is a dentist. (non defining relative clause)

My brother who lives in France is a dentist. (defining relative clause)