Paul's Grammar Corner
I have a cat which is purple.
I have a cat that is purple.
Which is correct?
Answer: Neither, I don't have a cat. My wife is allergic to them and her eyes pop out on stalks and turn red like large radishes. She forgets how to breathe and has to go to hospital. Besides, cats don't have owners, they have staff.
In fact, advice differs. Some say that both sentences are correct in British English. My view is that the second sentence is right. To differentiate them you need to know about relative restrictive clauses.
A restrictive clause is a clause containing important and essential information about the noun preceding it. In this case, use that. For everything else, use which. For example:
"F1 cars that use slick tyres go better in the wet". I warned you about English spellings back on page one, so don't moan. Here, the clause "that uses slick tyres" is essential to the meaning of the sentence. If you remove it you're left with "F1 cars go better in the wet", which is relatively meaningless. Better that what? In the first sentence the implication is that cars that use slick tyres go better than cars which don't. That's what the restrictive clause does.
"Which" would be used where no restrictive clause exists but where a non-restrictive clause might exist:
F1 cars, which are my favourite land vehicles, go very fast indeed.
Note two things: the non-restrictive clause (which are my favourite land vehicles) is surrounded by commas. Note also that if I remove that clause to leave, "F1 cars go very fast indeed", the overall meaning of the sentence is unchanged.
In this case always use "which". The commas are the clue.
This is as tough as this training will get, at least until we get to "who or whom", which coincidentally is next...