Okay, these have popped up in the blog before, but I don't think they were fully explained. We're all familiar with personal pronouns: I, me, my, he, she, it, etc. Relative pronouns are who, whom, whose, which, and that. They stand in for nouns or for noun phrases much in the same way that personal pronouns do, but there is one major difference. Take a look at this sentence, for example: “I ate a radioactive isotope, which turned me into the Hulk.”
What you're really saying is, “I ate a radioactive isotope. A radioactive isotope turned me into the Hulk.” However, the relative pronoun “which” is taking the place of that second, underlined “a radioactive isotope.” Relative pronouns, then, go a step beyond personal pronouns because they not only replace a noun phrase, but they relate (hence the term “relative pronoun”) two otherwise separate clauses.
The newly-dependent clause, “which turned me into the Hulk,” is what's called a relative clause. These come in two flavors: restrictive and nonrestrictive (or essential and nonessential). As such, they're governed by the same rules as all restrictive and nonrestrictive phrases in regards to the usage of commas, which you can check out in Jen's and Chan's posts on Sept. 27th.
Good luck with the test, guys!