Which vs. That
There is a very specific situation where the difference between which and that is very important. When adding details to a sentence that describes the subject, the meaning depends entirely on which word you use to link the descriptive clause to the rest of the sentence. Take these two sentences:
1) The book, which I read over the summer, was super interesting.
2) The book that I read over the summer was super interesting.
The differences between the two sentences is subtle but important to what they each imply.
In the first sentence, “which I read over the summer” is a nonrestrictive clause, meaning that the details it gives are not really important in clarifying or describing the subject (the book). You can think of it as an afterthought that isn’t really necessary. Here’s another way of writing out what the speaker meant:
The book was super interesting. By the way, I happened to read it over the summer.
Notice that, when using which inside a nonrestrictive clause, you need to add commas on each end of the clause to separate it from the rest of the sentence.
In the second sentence, “that I read over the summer” is a restrictive clause, meaning that the details it gives are necessary to the very definition/description of the subject (the book). In this case, it is clear to us that the speaker is specifically referring to a book that they read over the summer, whereas the which version of the sentence isn’t specifying the summer book, but rather mentioning that the speaker happened to read it over the summer.
The version with that implies that the speaker read several books and feels that it is necessary to specify that the book he is referring to was the one he read over the summer. When using that as a restrictive clause, you do not need to use commas to separate the clause from the rest of the sentence.
Try to combine these sentences using which or that. Notice how it slightly changes their meanings depending on which word you use to combine the sentences.
Ex. The cat is eight years old. I adopted it two years ago.
Which: “The cat, which I adopted two years ago, is eight years old.”
That: “ The cat that I adopted two years ago is eight years old.”
The rumor ruined my reputation. It claimed that I cheated on the test.
I can’t believe I ate the yogurt. It expired three weeks ago!
The red car is in my garage. I won it on a game show.