People freak out about commas. Please don’t. Yes, commas are hella confusing (the Chicago Manual of Style dedicates 59 distinct sub-sections to them, and even then there is ambiguity and opinion and wiggle room leftover), but knowing these four little things will almost entirely solve your comma problems.
1. The “Oxford”/”serial” Comma
This is technically the “list” comma: When you give a list, you put commas between individual items. E.g.,:
Go to the store and get eggs, pineapple, a ’57 Chevy, and enlightenment.
N.B. that the last comma (which I’ve put in red) is disputed; that lil fella is an “Oxford comma”. Some folks say it’s unnecessary (including, at least at one time, the AP Style Guide), preferring:
Go to the store and get eggs, pineapple, a ’57 Chevy and enlightenment.
But this can lead to hilarious ambiguity, such as this oft-quoted (and probably apocryphal) book dedication sorely in need of an Oxford comma:
This book is dedicated to my parents, Ayn Rand and God
—or this actual and verified sentence published in the Times of London a few years back:
FYI, serial commas apply to lists of adjectives, too:
You mean that fat, red, flabby car? It’s mine. Why?
2. The “if, then” Comma
“If . . ., then . . .” statements need commas:
If you don’t cut it the fuck out, then I’m going to freak the fuck out.
Where this one tricks people is that we often omit the then in an “if, then” statement—nonetheless, we still need the comma:
If you don’t cut it the fuck out, I’m going to freak the fuck out.
3. “That/which” commas
Rule of Thumb: “that” is almost never preceded by a comma, while a standalone “which” is almost always preceded by a comma:
You know that dog I hired? Turns out he has no idea how to use Excel, which is super annoying.
(So what is a non-standalone “which”? “Which” used in a phrase like “that which” or “in which”—in those cases, you don’t stick a comma before the “which,” because that would muck up the phrase.)
4. Commas by Ear
There are a ton of other commas (“parenthetical commas,” “conjunction commas,” “direct address commas,” etc., etc., ad infinitum, ad nauseam)—fortunately, there’s an easy way to figure out where to put them without learning a ton of new rules. Here’s the trick:
Commas indicate places where you take a brief pause when saying something.
So, there’s a really easy way to get your commas right most of the time:
Read it aloud; if there’s a place where you naturally take a half-pause or shift volume, then stick a comma there. If you don’t pause, then strike the comma—unless it’s one of the three situations listed above.
For example, say:
Dave <pause> why did you say that?
That pause is the direct address comma:
Dave, why did you say that?
Did you know that Nate <pause> the terrible drunk in my carpool <pause> is marrying my sister?
That’s the parenthetical comma:
Did you know that Nate, the terrible drunk in my carpool, is marrying my sister?
This fourth rule is the golden rule, since most of the first three types of commas are also marked in speech by a pause or volume/tone shift—but sometimes those commas can be subtle to the ear, which is why it’s worth knowing the first three rules. Rule #4 will keep you covered 90% of the time, while Rules #1–3 will help you catch the tricky 10%
We’re done here. Go forth, my children, and sin no more.
(Want more details? Start with the Purdue Owl on Commas, and then move on to the Chicago Manual of Style, if need be.)