“which” versus “that”¶
I suggest to stop saying “which” when we want to say “that”. The difference isn’t that difficult to understand. For example you say
“The bike that has a broken chain comes first”
when there are multiple bikes to repair, and you tell your colleague which one comes first. But you say
“The bike, which has a broken chain, comes first”
when there are multiple objects to store in the lorry, and you want the bike to come first because it has a broken chain.
Yet many people still seem to prefer using “which” also in the first case. They say:
“The bike which has a broken chain comes first.”
Please stop using this third form. Because it hurts once you have understood the difference. Because it obscures your message. Because it disturbs our communication.
A common mistake¶
Yes, it seems that in British English it is explicitly allowed to use “which” instead of “that”. But that’s only because the English language has a long history.
Rule¶
Look at the thing you’re talking about (the bike in our example):
If there are multiple bikes and your clause narrows down which of them you mean (a “restrictive clause”), then use “that” and no commas.
If there is only one them and your clause is an additional information about it, then use “which” and add commas around your clause.
2. If you are about to say “which”, but without a comma before it, then say “what”.
Examples of “wrong” usage¶
“This jar is not empty but contains a small amount of white sand which shifts on the bottom.” (Erin Morgenstern, Night Circus, p. 299)
“There are, it is true, works of recent Japanese literature which are relatively untouched by Western influence.” (Donald Keene, 1958, in Translator’s introduction to No longer human by Osamu Dazai)
“Django 5.0 brings a deluge of exciting new features which you can read about in the in-development 5.0 release notes.” (Natalia Bidart, September 2023, djangoproject.com)