This is a tale of coordinating conjunctions and subordinating conjunctions. What’s the difference? Coordinating conjunctions join two independent clauses; subordinating conjunctions join an independent clause to a dependent clause. Who cares? Well, the comma does. Because the comma has to know whether to jump in between those clauses.
So, first let’s look at the coordinating conjunctions. We have a handy little acronym to help us remember them: fanboys – for, and, nor, but, or, yet, so. When these words join two complete sentences, you have a compound sentence. Use a comma. “We wanted candy, but we settled for gum.”
Subordinating conjunctions (sometimes called dependent markers) join a dependent clause to an independent clause, and they don’t take a comma. Examples of subordinating conjunctions are: after, although, because, before, even if (or though), if, once, rather than, since, though, till, unless, until, when, where, while. A proper sentence using a subordinating conjunction would be, “We went to the store after we left work.” People sometimes put a comma in a sentence like this because they think they have two independent clauses (“we went to the store,” “we left work”), but turn the sentence around and it becomes obvious: “After we left work, we went to the store.” It’s pretty obvious “after we left work” is not independent.
There are some words that people think are coordinating conjunctions but aren’t, especially “then” and “because.” You see it all the time – “I picked up the trash bag, then I opened the trash can.” “I left the room, because he was driving me crazy.” This, my friends, is the dreaded COMMA SPLICE. Neither of these words join two independent clauses and neither gets a comma. How do you know? Neither is a FANBOYS coordinating conjunction.
It’s more obvious with “then.” To make the sentence correct, you’d say, “I picked up the trash bag and then opened the trash can,” or “I picked up the trash bag, and then I opened the trash can.” (To use that comma, you have to add “and” and have a subject and a verb on both sides of the “and.”)
But what about “because”? You can’t just remove the comma to correct a comma splice, right? According to Purdue, “because” is a dependent marker. (No, I’ve never heard of such a thing either.) http://owl.english.purdue.edu/owl/resource/604/01/. And I think that’s where the confusion lies. “He was driving me crazy” is a complete sentence, right? Not here. “Because” is a dependent marker that joins the clause that follows, turning it into a dependent clause. It’s not “he was driving me crazy”; it’s “because he was driving me crazy.” Saying it that way, you see that it’s dependent. So the correct sentence would be “I left the room because he was driving me crazy.” No comma.
(For an excellent discussion of why “then” isn’t a coordinating conjunction, check out http://grammar.ccc.commnet.edu/grammar/conjunctions.htm. They have a great sidebar in which they suggest you try moving the conjunction to other places in the sentence. If it works, it’s subordinating; if not, it’s coordinating. I can’t say it better than they did, so check it out.)