The words “which” and “that” have a long and tangled history together. I’d guess that most native English-speakers don’t recognize a difference between the words in most sentences.
The spaceship that looked like a tulip crashed into my swimming pool.
The spaceship which looked like a tulip crashed into my swimming pool.
Most native English-speakers wouldn’t bat an eye at either of those sentences, figuring that they mean basically the same thing.
And to be perfectly honest, just as most readers see them the same, so do most writers. So I wouldn’t recommend worrying about it very much in your everyday life.
So why should you care? Well…
Some readers do see a difference. So if you’re a writer, you should know that those readers will say that the second sentence is just wrong.
When these readers see the second sentence, they instantly stop thinking about whatever you’re writing about and start thinking about poor grammar and the lamentable state of the education system.
In other words, it might make sense to get it right.
[By the way, some guides mention that this distinction is practically eradicated in Britain. I recently read a British novel that used “which” instead of “that” in every instance. To my immense surprise, it actually started to annoy me.]
So how do you get it right? The short and slightly simplified answer is:
Use “that” when the phrase must remain in the sentence. Use “which,” with commas, when it could be left out.
What do I mean? Let’s go back to the examples above.
Here’s how a paragraph might lead up to statement number one (the “that” statement):
The fleet of 10,000 flower-shaped spaceships arrived in a blinding flash of color. Each ship looked like a different flower. And then something happened. The spaceship that looked like a tulip crashed into my swimming pool.
The point of the sentence is that a very particular spaceship did something. The question of the ship’s shape is an important one.
If you took out “that looked like a tulip,” you’d be left with:
The spaceship crashed into my swimming pool.
And people would ask, “which spaceship?” Then they’ll say, “you said that there were 10,000 ships, so you have to be more specific than just saying “the spaceship” without telling us which one you mean.”
That is definitely what they’d say.
Now here’s a paragraph containing a “which” phrase:
The fleet of 10,000 flower-shaped ships arrived in a blinding flash of color. Suddenly, a spaceship spun wildly out of control. The spaceship, which looked like a tulip, crashed into my swimming pool.
In this case, you could safely remove the phrase in question. We’ve already introduced and specified the ship in question, so the bit about the shape is just confection.
In short, disposable stuff gets commas and a “which.” Essential stuff gets a “that.”
Is this making sense? Should we stop worrying about this distinction?