restrictive and nonrestrictive clauses

Penguins, which I hate, live in Antarctica.
Penguins that I hate live in Antarctica.

Above, you see two different adjective clauses in two different sentences. The adjective clause "which I hate" is a nonrestrictive clause. Why? Because this clause is basically a parenthetical, "by the way" sort of remark that isn't essential to the sentence's main thrust, which is the claim that penguins live in Antarctica. You can safely remove "which I hate" and still get the sentence's main idea. If you remove "which I hate" and merely say that "Penguins live in Antarctica," you're talking generally about all penguins. If you add "which I hate" back in, you're still talking generally about all penguins. Your focus on penguins hasn't been restricted, i.e., it hasn't been narrowed.

Look at the second sentence. The adjective clause "that I hate"—which has no commas surrounding it—serves a crucial function because it very specifically identifies a set of penguins: the penguins that the writer hates. Maybe the penguins fucked the writer over at some point, and the writer never forgave them. Whatever the case, there are some penguins in Antarctica that the writer hates. Taking out "that I hate" would make a clear sentence ambiguous. So "that I hate," with its lack of commas, is a restrictive clause: it restricts (i.e., narrows) your focus to something particular, and the information is essential to the core meaning of the sentence. There is no casual "oh, by the way" sense here.

When people talk about the difference between the relative pronouns that and which, this is the technical distinction between them: use which with nonrestrictive clauses and that with restrictive clauses. And use commas with your nonrestrictive clauses; depending on context, you might need one or even two. As a rule of thumb, put a comma before your which whenever you use it as a relative pronoun, and you won't go wrong.

Some folks will say that that and which are often interchangeable. This isn't totally false (especially for informal spoken English), but it's only provisionally true. Merken Sie gut!

I'll be writing more about this as I continue my series on commas.