Some people, grammar authorities or not (mostly not), pooh-pooh the notion that distinguishing between that and which makes a whit of difference in people’s comprehension of what one is saying or writing.
This is probably true, as it is also true with who and whom. Most people will never know or care about the difference or stumble over one’s meaning if the two are interchanged.
That being said, there really is a difference between that and which, which is this: One is essential and one is not; one is restrictive and one is not.
The essential and restrictive one is that. When you want your clause to be integral to the meaning of your sentence, you use that (or who for people, which is neat because who can be both restrictive and non-restrictive).Â For instance:
“Cats that are declawed are defenseless.” If you remove the that clause, the sentence no longer has any meaning, at least not the intended meaning. Therefore this is an essential or restrictive usage. (Note also that you never use commas with that.)
“The neighborhood cat, which has been declawed, visits us frequently.” Here, if you remove the which clause, the sentence still retains its essential meaning. The which clause in this usage acts almost like a parenthetical expression, and thus is called non-essential or non-restrictive. Note here too that you must set the clause off with commas because of its parenthetical nature.
Okay, so go ahead and confuse using the two. Thomas Jefferson and Benjamin Franklin did, and they certainly found a huge audience of readers.
Maybe back then no one cared.Â Come to think of it, virtually no one cares now either, but there still is a distinction between that and which. I guess I care since it’s just as easy to be grammatically correctly as it is to be lazy or slovenly.