Don’t worry if you’ve never heard of “restrictive” and “nonrestrictive” clauses. Knowing technical grammar terms is rarely important. Understanding how they function in the real world, on the other hand, is invaluable.
Part 1: When “That” and “Which” Are Interchangeable
“It was one of those pictures which are so contrived that the eyes follow you about when you move.” (Emphasis added.)
In the above sentence, “which” could have been “that,” and it also would have been correct.
In simple terms, without using a comma before “which,” “that” and “which” are interchangeable. Just make sure not to use “which” to refer to people; it is okay to refer to people with “that,” e.g., I know a girl that can speak 12 languages, but “who” is considered the better choice.
So far, so good?
Part 2: When They’re Not Interchangeable
“That” is used to introduce a restrictive clause. This is just a fancy way of saying that if you get rid of the clause, the meaning of the sentence will change.
Take a look at the following example:
See how we’ve completely changed the meaning of the sentence? The original sentence meant that only the games that demean women should not be sold at toy stores. But by deleting the restrictive clause, we are now saying that all video games should not be sold at toy stores.
Therefore, we use “that” to provide essential information (details that cannot be deleted) that specify exactly what thing or person you are talking about.
On the other hand, “which” is used to introduce a non-restrictive clause. The non-restrictive clause is “sandwiched” between commas. “Which” functions like parentheses:
The above sentence means that all video games, by their very nature, demean women. This is much different in meaning from the previous sentence that used “that demean women.”
Therefore, be careful when you choose whether to use “that” or “which”: choosing the wrong one can make you express an idea that you didn’t mean to.
Here is one more set of examples:
- The spinning chair that I bought at IKEA is broken. = The only spinning chair that is broken is the one that I purchased at IKEA; the other spinning chairs (from other stores) are fine.
- The spinning chair, which I bought at IKEA, is broken. = I have one spinning chair. I bought it at IKEA. It’s broken.