Anyone who either (a.) doesn't care at all about grammar and doesn't want to read a post about it, or (b.) cares enough about grammar that, once they learn a new rule, they'll always notice it, should probably stop reading now, because I'm about to write about the difference between that and which. I'm writing about this, not because I think it's important that everybody always get it right, but because I have personally had a lot of reading experiences worsened by the knowledge of the correct usage of which. The reason is that people use the word which all the time, because it's one of those words that just sounds like it ought to be correct more often. Loads of people just assume that they're using that wrong, so they start substituting in whiches all over the place.
Here are some hypothetical examples of probable misuse of the word which:
- I like cheese which is aged for more than ten years.
- Don't open any doors which look like they might be holding back ghosts.
- I have an aunt which won't stop texting me during dinner.
Here's what those sentences mean, if the use of which is correct:
- I like cheese, and anything called cheese must have been aged more than ten years.
- Doors, by definition, look like they might be holding back ghosts, so you should not open them.
- Aunts are nonhuman creatures that text me during dinner; I only have one.
One can account for those problems by switching out the whiches with thats and whos as appropriate:
- I like cheese that is aged for more than ten years. (I like old cheese better than new cheese.)
- Don't open any doors that look like they might be holding back ghosts. (It's fine to open doors if there probably aren't any ghosts behind them.)
- I have an aunt who won't stop texting me during dinner. (My one particular aunt keeps texting me during dinner. Furthermore, she is a human being, and a person can be an aunt even if they don't text me during dinner.)
Now, there's not much to gain by knowing this. Like most grammatical nitpicks, the correct meaning is obvious something like 95% of the time, and for another 4.9% the confusion can be settled just by talking about it a little longer.
Unfortunately, now that you've learned this, at least some of you will never be able to unlearn it, and dozens of teachers, orators, essayists and bloggers whose grasp of language you once admired will suddenly become very annoying to read.