Nothing that follows is meant to substitute for the nuanced explanations of what's usually called a writing handbook, the sort of book that college students purchase in a first-semester writing course. These five rules though have the virtue of being manageable, which is difficult to say of a 1,000-page book. In each paragraph that follows, the sentences illustrate the punctuation rule involved. Note that I'm avoiding almost all grammatical terminology. Instead, I'm emphasizing a small number of sentence patterns.
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.
Any element, big or small, that interrupts the movement of a sentence should be set off with commas. This sentence, like the first, also has an element set off with commas. An extra element at the end of the sentence should also be set off with a comma, as I'm showing here.
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. John, Paul, George, and Ringo.
(There's no consensus about using a comma before the final item — the so-called "Oxford comma" or "serial comma." Keeping that comma seems to me the better choice, simplifying, in one small way, the problems of punctuation. If you always put the comma in, you avoid problems with ambiguous or tricky sentences in which the comma's absence might blur the meaning of your words.)
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.
Complete sentences that are joined without a coordinating conjunction need a semicolon instead of a comma; the semicolon shows the end of one sentence and the beginning of the next. Semicolons 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 semicolon, and another complete sentence:
[complete sentence]; [complete sentence].Some examples:
Your argument is persuasive; it addresses every objection I had.The second pattern to look for involves a complete sentence, a semicolon, a connecting word or phrase, a comma, and another complete sentence:
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.
[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.How can you tell whether you have two complete sentences or one sentence with an additional element at its end? With an additional element (something less than a sentence in itself), the parts of the sentence can be switched and still make sense:
The proposal is flawed; thus, we're sending it back for revision.
She did well in the class; in fact, she did much better than she had expected.
I'll go to work, even though I'm sick.But with a second complete sentence and a word or phrase such as instead, thus, or in fact, the parts cannot be switched and still make sense.
Even though I'm sick, I'll go to work.
A complication: when you can switch parts, a comma will sometimes be necessary and sometimes not. The best way to judge is to consider whether the element at the end is necessary to the meaning or something extra. Consider these examples:
Why did you bring an umbrella?In the first exchange, the words “because I thought it would rain” are crucial to the meaning. In the second exchange, they’re not.
I brought an umbrella because I thought it would rain.
What did you bring?
I brought an umbrella, because I thought it would rain.
I think of this kind of comma as analogous to seasoning — sometimes you need it; sometimes you don't. (And at this point, very few people are liking to be thinking about the choice in terms of outright error.)
These are the basics of punctuating sentences with commas and semicolons. 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.
Colons, by the way, function as arrows or pointers: see what I mean?
How to punctuate more sentences