TYPICAL ERROR Samantha who hurt her ankle playing hockey had to go to the X-ray department to have her ankle X-rayed. The nurse who met us at reception thought she had broken it.
CORRECTION Samantha, who hurt her ankle playing hockey, had to go to the X-ray department to have her ankle X-rayed. The nurse who met us at reception thought she had broken it.
EXPLANATION Notice that in the example there are two adjectival clauses:
who hurt her ankle playing hockey
AND
who met us at reception.
There is a fundamental difference between the two. While the first clause could be left out, the second clause is essential; without it we would not know which nurse is being referred to. We are not talking about just any nurse but the nurse who met us at reception. The clause identifies a specific nurse and so we call it a defining adjectival clause.
If we read the first sentence without the adjectival clause, i.e.
Samantha ... had to go to the X-ray department
we see that it still works because the clause does not define Samantha; it just adds information. We thus call this sort of adjectival clause a non-defining adjectival clause.
Note that commas are always inserted around non-defining clauses but they are never used with defining clauses because we read the noun and its defining clause as one entity, as in The nurse who met us at reception...
Incidentally, who is used with both types of clauses when referring to people. Furthermore, non-defining clauses should never be introduced by that; that is used to introduce defining clauses. While which can be used for either, it is better to reserve which for introducing non-defining clauses, which makes interpretation easier in sentences that contain both defining and non-defining clauses, as in the following sentence:
The Great Wall of China, which is often wrongly said to be the only structure made by humans that can be seen from space, wends its way for 2400 kilometres across China.
In each of these pairs of sentences, one contains a defining clause and the other a non-defining clause. Add commas to the sentence containing the non-defining clause.
aAdobe bricks which are made from sand, clay and fibrous material such as straw and animal dung have been used for building for thousands of years. Spaniards who travelled to Mexico and Peru in the sixteenth century took the technique back to Spain.
bDawn Fraser who won eight Olympic Games medals and eight Commonwealth Games medals was named female athlete of the 20th century by the Australian Sporting Hall of Fame. She is a sporting personality who has achieved iconic status in Australia.
cThe first human who travelled into outer space was Yuri Gagarin. Ironically, he was to die in a plane crash which happened just seven years after he had survived space travel.
dMy mother who has never been overseas is looking forward to her trip to New Zealand. Two friends who are accompanying her have been there before.
eThe television program that has the best ratings for that time slot is So You Think You Can Dance. Even my brother who has never learnt to dance watches it religiously every week.
