People seem to use commas by extremes: either commas are everywhere or they are completely absent. Heck, I even plopped one into that last sentence and had to pull it out in the first editing pass. Commas have two purposes: they create a pause in the text or they separate part of the sentence for clarity.
Let’s look at the nefarious comma splice. A comma splice is when two phrases that separate grammatically complete sentences are connected erroneously by a comma. Like this:
The dog ran back to us, he was chewing an unfamiliar tennis ball.
This needs to be two sentences using a period, or the single sentence needs a semicolon instead of the comma.
The dog ran back to us. He was chewing an unfamiliar tennis ball.
The dog ran back to us; he was chewing an unfamiliar tennis ball.
Either way is fine; you choose, depending on where you want the emphasis or focus to be. The two separate sentences make perfect sense just as they are, each providing emphasis on the action within. Using the semi-colon, the two halves are linked, but are grammatically discrete. The emphasis in this linked version seems to be more on the chewing of the ball than the running back. You could also recast slightly and use a perfectly good subordinate clause offset by a comma.
The dog ran back to us, chewing an unfamiliar tennis ball.
This sentence puts nearly equal weight on the dog’s running back and his chewing a tennis ball. This sentence reads much like the semi-colon version, above, as far as focus or emphasis. You could turn the example sentence around to move the emphasis, but you don’t get rid of the need for a comma because the chewing portion is still a subordinate clause.
Chewing an unfamiliar tennis ball, the dog ran back to us.
Putting the subordinate clause first places clear emphasis on it.
One test you can make for comma appropriateness is to read the sentence aloud. If you need to pause in your reading for the sentence to make sense, you need punctuation. A full stop gets a period, a partial stop gets a semi-colon, and a little pause gets a comma.
Let’s look at prepositional phrases. (There’s a previous blog on prepositional phrases, if you’re interested.) You need to offset the prepositional phrase with a comma for the same reason as the subordinate clause—it IS a subordinate clause.
In June of this year, version three will be released to manufacturing.
Go ahead. Try to read that sentence without a little pause after year, and see if it still makes sense to you. You can’t. Alternatively, you could leave the whole prepositional phrase off and the sentence still makes sense. “In June of this year” doesn’t offer any information that can stand alone, so it is a subordinate clause.
Commas also get used in a series to separate the elements of the list. Just like the subordinate clause, the comma causes a little pause when you read the list aloud. Some style guides require a comma before the conjunction (and, or, nor) and final element, and other style guides don’t. This final comma is called a “series comma,” even though the whole thing is a series anyway. (It can also be called an Oxford or serial comma.)
The salad contained spinach, soy beans, onions, mushrooms, fennel, boiled egg, and a balsamic vinaigrette.
Notice that there’s no comma before the first element in the list.
I like the series comma because Chicago Manual of Style likes it, but it also lets me make a little pause after the penultimate element, just like all the rest. I’m big on parallelism like that.
You also need a comma before a non-restrictive clause.
For the majority of users whose native language is not English, the product’s Help files are insufficient.
For the majority of users, whose native language is not English, the product’s Help files are insufficient.
In the first sentence, there is a subset of users who don’t speak English as a first language. The clause is restrictive because it narrows the spectrum of users in a specific way. In the second sentence, most of the users don’t speak English. It’s a non-restrictive clause because it further describes the users as a whole group—it doesn’t narrow the group.
The same holds true for the ever-tricky that/which debate. Most of the time, “which” is non-restrictive.
The book, which was full of mistakes, lay on the table.
In this sentence, the point is that the book is on the table. You might recast it as:
The mistake-laden book lay on the table.
The book lay on the table.
Both have the same effect, although the first sentence is more descriptive than the second. If you’re trying to separate one book out from a bunch of other books by pointing out which one it is, use a restrictive clause (with no comma).
The book that was full of mistakes lay on the table.
The point of this sentence is where the book is and which book it is, so it’s restrictive. You can’t recast the sentence to have the same effect without rearranging it significantly.
The book lay on the table; the book was full of mistakes.
You have to rename the noun (the book) in the second half, because otherwise the table might be thought to have mistakes. There will be a whole other blog on restrictive and non-restrictive, but my point here is to show you the comma’s part. For now, you can probably just remember that “which” needs a comma and “that” doesn’t.
Other reasons to use commas include separating place names (San Francisco, California), separating parts of a date (January 3, 2004), separating a long series of adjectives preceding a noun (the happy, go-lucky little boy), offsetting important but parenthetical comments or information (the frog, always ready for lunch, was happy), offsetting a quotation (he said, “that simply won’t do”), and offsetting an adverbial clause (clearly, you are correct). Comma Comments, Part 2 covers those (available in a few days in this blog).