My father and I were sitting peacefully at breakfast the other day when I got a text that simply read, "What's the difference between 'which' and 'that'?" Now, my dad doesn't like when his offspring text in front of him, so I usually try to include a disclaimer when I do it so he doesn't clear his throat and give me one of those looks. I started scribbling sentences onto a napkin.
"Someone wants to know the difference between 'which' and 'that,'" I clarified, alternately texting and jotting. "How do I explain that?"
Something you should understand about this particular parental unit of mine is that he's not exactly...a words person. He's not the first person you would go to if you wanted the definition of an antecedent, or even to figure out what the subject of a sentence is. So the fact that this conversation even started is pretty strange all by itself. My quick answer was this:
"Um...you use a comma before which?"
But the real answer is slightly more complex. Thus began an amusing back and forth between my dad and me on how to explain the difference between 'which' and 'that.'
"It's like...describing a coat," I said, writing two sentences on the napkin. "'I bought a coat that was red' and 'I bought a coat, which was red.' One is necessary; one isn't. You see, when you use an appositive..." I think I lost my dad at that point. So for his benefit and yours, here's a crash course on "which" vs. "that."
As it turns out, the appositive doesn't actually come into play. But we are dealing with what are called essential and nonessential clauses. They mean pretty much what they sound like. The nonessential clause is a further description of the subject of the sentence. These clauses are always set off by commas: "The artist, an Irish man with a beret, liked to paint cats." (You can't tell but those commas are italicized too.)
An Irish man with a beret describes "the artist," which is the subject of the sentence. The phrase an Irish man with a beret is the nonessential clause in this situation.
There are also essential clauses. These are similar to nonessential but they differ in that they are actually needed to make the sentence clear. For example, "The lady dancing naked in the street is my old babysitter."
The difference between the two examples is this: There could be many ladies around in the second example, but the speaker is interested only in the one dancing naked in the street. If the speaker said only that "the lady is my old babysitter," the listener could be confused as to which lady was being referred to. In the first example there's really no question about the identity of the artist. The speaker is simply offering some additional detail about the man who likes to paint cats.
This idea of essential and nonessential information carries over when we use words like "which" and "that." These are what I call describing words, and you use them to add more information to your sentences. They're pretty simple to differentiate; use "that" with essential clauses, and "which" with nonessential clauses.
I like to ride yaks, which are big and hairy.
I like to ride yaks that are big and hairy.
You can see the difference. In the first example which are big and hairy is a nonessential clause. You don't need to know that yaks are big and hairy to figure out what the speaker is talking about; it's just a nice bit of information so you can better visualize the yak. In the second example that are big and hairy is essential to the sentences. The speaker is clarifying that he or she prefers to ride the big and hairy yaks instead of the ones that are small and hairless.
My father has his own method for remembering the different situations in which to use "which" and "that." He said to me, "So...if you have a catalogue, and you need to describe to your customers essential information like the color of the coat, you would use 'that.' Because your customers need to know what the products look like." I didn't really get where he was going with that, but if that helps you to remember it better, by all means use that example instead and ignore all above pictures and explanations.