‘That’ or ‘which’?
Should you write which or that? It’s a common dilemma but, fortunately, it doesn’t usually matter which one you choose.
In British English, which and that are equally correct in many cases – as in this example:
“He hid the vase which he broke. ”
“He hid the vase that he broke.”
As the Oxford Dictionary on ‘that’ or ‘which’ explains, it’s to do with things called restrictive relative clauses and non-restrictive relative clauses. In the sentence above, that and which introduce a restrictive relative clause. The clause contains essential details about the vase – the fact he broke it – that it wouldn’t make sense to leave out. You can use that, which, whose, who, or whom to introduce a restrictive relative clause. And you don’t need to put a comma before them.
That brings us to non-restrictive relative clauses. These contain optional information that you could leave out without affecting the meaning or structure of the sentence. You can use which, whose, who, or whom to introduce a non-restrictive relative clause, but not that. For example:
“He enjoyed reading posts about grammar on the blog, which also featured photos of kittens. ”
“I’m not going to be able to make your party tonight, which is a shame.”
You put a comma before which, whose, who, or whom in this case because you're introducing additional, rather than essential, information.
All sounds a bit complicated? Just follow The Economist’s style advice and you won’t go wrong: ‘which informs, that defines’.