Rule one If your sentence begins with an introductory element, put a comma after it. Even if it’s a short element, put a comma after it. In time, you’ll be putting this comma in without having to think about it.
Rule two Any element which interrupts the movement of the sentence, whether it’s big or small, should be set off with commas. This sentence, like the first, also has an element set off with commas. An element that appears at the end of the sentence should also be set off with a comma, as I’m showing here.
Rule three Items in a series should be separated with commas. What do I mean by “items in a series”? Wine, women, and song. Life, love, and laughter. Crosby, Stills, Nash, and Young.
Rule four Complete sentences that are joined by a coordinating conjunction (and, but, for, nor, or, so, yet) need a comma before the coordinating conjunction. That might seem obvious, but this comma frequently gets left out. Putting it in makes a sentence more readable, and any reader appreciates that.
Rule five Complete sentences that are joined without a coordinating conjunction need a semi-colon instead of a comma; the semi-colon shows the end of one sentence and the beginning of the next. Semi-colons are often followed by a connecting word or phrase; however, a connecting word or phrase is not necessary. Sentences joined with only a comma are called comma splices; they’re among the most common errors that come up in college writing.
(Note: In the next-to-last sentence in the previous paragraph, there’s a comma after however because it’s an introductory element in the second sentence.)
Fixing comma splices requires familiarity with two recurring sentence patterns. The first involves a complete sentence, a semi-colon, and another complete sentence:
[complete sentence]; [complete sentence].
Your argument is persuasive; it addresses every objection I had. His research paper is plagiarized; he is going to fail the class. The novel is a relatively recent literary form; it’s not nearly as old as epic poetry and lyric poetry.
The second pattern to look for involves a complete sentence, a semi-colon, a connecting word or phrase, a comma, and another complete sentence:
[complete sentence]; [word or phrase], [complete sentence].
(Again, the comma after the connecting word or phrase is appropriate as that word or phrase is an introductory element in the second sentence.)
I decided not to take the job; instead, I’m going to graduate school. The proposal is flawed; as a result, we’re sending it back for revision. She did well in the class; in fact, she did much better than she had expected.
How can you tell whether you have two complete sentences or one sentence with an interrupting element at its end? With an interrupting element (something less than a sentence in itself), the parts of the sentence can be switched and still make sense:
I’ll go to work, even though I’m sick. Even though I’m sick, I’ll go to work.
But with a second complete sentence and a word or phrase such as instead, as a result, or in fact, the parts cannot be switched and still make sense.
Those are the basics of punctuating sentences with commas and semi-colons. I know from working with many students that any writer can get better when it comes to punctuation. The key is the ability to recognize a handful of familiar patterns. Look for the patterns in your sentences, and you too can get better. With some practice, you’ll be able to see the parts of your sentences falling into place, and punctuating correctly will become, believe it or not, a habit, one that you’ll be happy to have acquired.