When to use “which” or “that”?
This is a grammar conundrum which is specific to the US, and that confused me for quite some time myself. If you’re in the UK or elsewhere that uses UK style, you probably don’t even need to read this post, as it will simply confuse you. Just keep doing what you’re doing and you’ll be fine.
For all you United States writers, heads up and pay attention!
Many people feel “which” and “that” are interchangeable. I used to think so, too, until I did some research and discovered there is indeed a difference. The usage difference stems from whether or not the information following which/that is necessary to the sentence (nonrestrictive) or unnecessary (restrictive). (I don’t like the terms nonrestrictive and restrictive, because to me they are backwards and I get them mixed up. So I’ll stick to necessary or unnecessary, which are more understandable.) If the clause following which/that is unnecessary to the sentence structure, then you surround it with commas and use “which.” If it’s necessary to the sentence, you don’t offset it with commas and you use “that.”
This difficult subject is one that is taught in our school.
This is correct, because the phrase or clause “that is taught in our school” is necessary to the sentence. It could not be removed from the sentence without it rendering the sentence grammatically incorrect. It doesn’t need to be set off with commas, and the use of “that” is appropriate.
If you wanted to use “which,” you’d have to rework it this way:
This subject, which is one taught in our school, is difficult.
In this example, the phrase “which is taught in our school” is unnecessary to the sentence structure. Therefore, it is surrounded by commas and you use the word “which.” You could remove the clause (not Claus!) and be left with “This subject is difficult,” which is grammatically correct and a full, complete sentence in itself.
Don’t confuse information that is necessary to your story with information that is necessary to the sentence structure! 🙂
For the rule we’ve just discussed, what do you think of these sentences?
The dog that was black barked incessantly all day.
I wanted to climb the tree which is over there.
My computer, that used to be Robert’s, is now broken.
My bathroom which was recently redone is now blue.
We’re Dun for today, so keep on Writin’!
Owner, Adirondack Editing