Before you learn what defining and non-defining relative clauses are, you need to know the following:
| Relative pronoun | Use |
| Who / that | People |
| Which / that | Objects, places, animals |
| Whose | Possessions |
| Relative adverb | Use |
| Where | Place |
| When | Time |
| Why | Reason |
What is the difference between defining and non-defining relative clauses?
Defining clauses are clauses that provide information that is important to understand the sentence. They are introduced with: who, that, which, whose, whom, why, when, where and do not need a comma.
That’s the park where we used to go as children.
She is the girl who taught me how to play the flute.
They are the reason why the project didn’t work out.
Non-defining clauses are clauses that provide additional information that is not essential to understand the sentence. They are introduced with: who, which, whose, whom, when, when and a comma is necessary. In addition, if the relative clause is not added, the meaning would still be the same.
Bob, who is an engineer, owns a company.
That girl, who is driving a classic motorcycle, is my neighbour.
Carlos, whose house is next to mine, is an English teacher.
Read more about English grammar here.