If a relative clause (an adjective clause) gives necessary information which is essential to the meaning, it is called a defining relative clause. The clause in not put in commas.
He is the actor who won an Oscar. [1, p.104]
A non-defining relative clause gives extra information and is not essential to the meaning of the main sentence. The relative clause is put in commas.
Tim, who doesn’t like to study, failed his test again.
London, which is the capital of England, attracts many foreign visitors. [1, p.104]
“It was the first time he had ever had breakfast in bed. First of all he had trouble with the grapefruit. Every time he pressed it with his spoon a long stream of juice shot up and hit him in the eye, which was very painful.” [2, p.38]
“When he looked round everything seemed very confused. A number of people were gathered round the fat man, who was sitting on the floor rubbing his head.” [2, p.46]
Sources:
- Evans, V. and Doodley, J. (2011). New Round-Up 5. Pearson Education Limited.
- Bond, M. (1992). A Bear Called Paddington. HarperCollinsPublishers.