1 A relative pronouns introduces a relative clause
1.1 that, which, who, whose, whom, where, when and why
2 In the sentence 'The golfer who attended
the annual club dinner had won two
previous championships', the sentence
tells us which golfer we're referring to. Of
all the golfers it was the one who
attended the dinner. So, it's a defining clause.
3 In the sentence 'The golfer, who attended the annual club
dinner, had won two previous championships.', it tells us
that there's likely to be only one golfer in question. The fact
that he attended the annual dinner is just extra information.
So, it' a non-defining clause.
3.1 The sentence still makes sense if you leave the
extra information out.
4 In defining clauses, the relative pronoun
can be omitted if it's an object.
4.1 Ballet and classical dance
techniques can be applied to other
sports (which) spectators find
pleasing to the eye.
4.2 It can't be omitted if it's the subject:
'Ballet and classical dance techniques
can be applied to other sports which are
also pleasing to the eye.'
5 In defining clauses, WHICH can be replaced for THAT.
5.1 THAT cannot be used after a preposition in
a relative clause. In this case, use WHICH
or WHOM when referring to people.