Americans fear and loathe the semicolon. This is likely because they simply don't understand how it's supposed to function. The very shape of the semicolon may be somewhat disconcerting as well: there's something perverse, off-kilter, and a bit unnatural about the way that that comma seems to be lurking salaciously beneath that period. Even the name "semicolon" sounds as if someone has done violence to someone else's intestines.
For all its apparent malevolence, however, a semicolon is a rather simple, straightforward creature. It plays two principal roles:
1. It separates independent clauses.
2. It acts as a "super-comma."
Poets and other creative folks might wish to add one or two other nifty features to the semicolon, but for my purposes, it's enough to talk about the above two functions which, together, cover about 99% of the instances in which you'll see and use this punctuation mark.
1. The Semicolon Separates Independent Clauses
When I taught the use of the semicolon to high schoolers in my previous job at YB, I often had to backtrack. The dialogue normally went like this:
ME: So! Semicolons. A semicolon separates two independent clauses. With me so far?
STUDENT: Uh... sure.
ME: You don't sound so sure. You know what an independent clause is?
ME: Do you know what a clause is?
At that point, I would just nod grimly and explain what a clause is, what dependent and independent clauses are, and what a semicolon in action looks like. So I'll do the same for you. Let's start with clauses.
A clause is an element of a sentence that contains a subject and a verb. In fact, a clause can be a sentence (a sentence is essentially a complete thought). Here are some examples of clauses:
Sheila couldn't stop staring at Jenna's amazing nipples.
Blofeld contemplated the humping rats.
Not to be outdone, Richard sharted.
He sharts with suspicious frequency.
Karl, no friend of the fascists, declared his love of Neapolitan ice cream.
If I had known you were going to wear that glowing thong, I'd have worn my lightning bra.
My testicles whisper to each other in French.
Although Max had trouble putting on the condom, the condom had no trouble slipping off during sex.
If you show anyone that photo, I'll sell you on Craigslist.
I love you because you have eleven fingers.
In each of the above sentences, you can find the clauses by stripping the sentences down to their skivvies, i.e., to their simple subjects and simple predicates. To wit:
Sheila... couldn't stop
I... had known + I... would have worn
Max... had + condom... had
you... show + I... will sell
I... love + you... have
So what's an independent clause, then? An independent clause is a clause (subject + predicate) that can stand on its own. All sentences, because they're complete thoughts, contain at least one independent clause. By this reckoning, Jesus wept is an independent clause: it's a complete thought, understandable without elaboration.
A dependent clause, then, is a clause that CANNOT stand on its own because it's introduced by a subordinating conjunction such as because/since, if, although, as if, provided (that), even though, etc.
Imagine that I stare meaningfully into your beautiful eyes and begin to say:
"Because you have eleven fingers—"
—then get shot through the heart with a crossbow bolt. You're left hanging, right? I haven't completed my thought, which leaves you in turmoil: "Huh? Because I have eleven fingers, what, Kevin? WHAT??" It's the because that makes the thought incomplete: a subordinating conjunction, like because, introduces a subordinate clause, and a subordinate clause is the same thing as a dependent clause. It can't stand by itself.
Now you know what a clause is (it's got a subject/noun and a predicate/verb), and you know the difference between a dependent and an independent clause. I think we're ready to get back to semicolons.
As you'll recall, a semicolon separates independent clauses. This means YOU SHOULDN'T USE A SEMICOLON WITH A CONJUNCTION. Don't use it with subordinating conjunctions (like the aforementioned although, if, because, etc.), and don't use it with coordinating conjunctions (like for, and, nor, but, or, yet, so: FANBOYS), either.
So the following sentences are all incorrect:
Bill knew a lot about the Internet; but he still paid for online porn.
If you tug my hair again; I'll crush your two balls into one big ball.
Although Clarence knew better than to stare at his partner's ass; he couldn't help himself.
Janet was the fastest; because she had the smallest boobs.
In almost every case above, the correct punctuation would be a comma. The lone exception is the sentence about Janet of the tiny boobs: because the dependent clause comes last, there's no need for a comma.
Here are some sentences featuring correct use of the semicolon:
Richard sharted; Jesus wept.
Elaine belched; Jasper farted.
I love you for your eleven fingers; you love me for my twelve.
Karl declared his love of Neapolitan ice cream; hearing this, my testicles whispered excitedly in French.
Hobbits like racy magazines with naughty woodcuttings in them; Gollum prefers to watch "Two Girls, One Cup" over and over again.
Semicolons connect clauses that have some kind of organic relationship to each other—maybe a contrast, maybe a similarity, but there's some notional connection. Try to avoid using a semicolon to stitch together two completely unconnected thoughts:
Godzilla's power and speed were amazing; I like pizza.
2. The Semicolon Acts as a "Super-comma"
Normally, the items in a list are separated by commas; the comma just before the "and" is called a serial comma or an Oxford comma. Examples:
Jason, since you're off to Wal-Mart, please bring back some superglue, an enema kit, and a crossbow.
Batman's enemies list includes the Joker, the Penguin, and carbs.
What do butt plugs, dildos, and unicorns have in common?
My favorite TV shows used to be "24," "Battlestar Galactica," and "House."
But what if the items in your list already contain commas?
Jack, my best friend
Jill, his wife
Bert, my ex-boyfriend
That's where the semicolon comes in handy! Like a sort of "super-comma," it separates list items that already contain commas. To wit:
Susan! For our wedding, I want to invite Jack, my best friend; Jill, his wife; and Bert, my ex-boyfriend. That OK with you?
Some other examples of the super-comma in action:
The new Star Wars movie will feature Mark Hamill, digitally rejuvenated; Sir Alec Guinness, brought back from the dead; and Fritz Hoffenpenis, the new voice of molluscan Jedi Master Millennium Falos.
My favorite X-Men are Wolverine, played by Hugh Jackman; Professor X, played by Patrick Stewart; and Magneto, played by Gandalf.
Gatsby wanted three things in life: Daisy, the object of his lust; fame, the culmination of his dreams; and a third testicle, the answer to his troubles.
And now... your quiz. Look at the punctuation in the following ten sentences and determine whether each sentence, as it's written, is CORRECT or INCORRECT. The answer is written between the brackets right after the sentence; highlight to reveal.
1. Although I find your eerily spherical boobs appealing, my heart belongs to another.
[CORRECT; no semicolon because these AREN'T two independent clauses. "Although" introduces a dependent/subordinate clause.]
2. The Nameless Gunfighter was wanted by Tuco; Samwise Gamgee; and Ed Grimley.
[INCORRECT; the items in the list don't contain commas, so there's no need for any super-commas.]
3. If I want your opinion, I'll suck it out of your soul.
[CORRECT; no semicolon because there's only ONE independent clause. (The "if" clause is a dependent clause.)]
4. I know you want children; but what if they end up with three heads, like me?
[INCORRECT; avoid using a semicolon with conjunctions like "but."]
5. The company of adventurers was led by Gandalf, the wizard, Aragorn, the future king, and Bilbo, the hobbit.
[INCORRECT; the list items have commas, so a brace of super-commas is necessary.]
6. When you reach Hell's Canyon, turn left, you'll see Satan's Abode about twenty miles after the turn.
[INCORRECT; this error is what we call a "comma splice," i.e., a comma (after "left") is being used where a semicolon should be.]
7. Because you ate the last Rocky Mountain oyster, my child will starve.
[CORRECT; no semicolon because there's only ONE independent clause.]
8. Gingerly patting his crotch after the grenade blast, Alex noticed that something had gone missing.
[CORRECT; the "Gingerly" locution isn't a clause at all: it's a modifier. Semicolons separate two independent clauses, so no semicolon was needed here.]
9. When Janeane farted and that hamster flew out; Josh knew it was over.
[INCORRECT; the "When" introduces a subordinate (dependent) clause; semicolons separate two INDEPENDENT clauses.]
10. Ever since Katniss decided to take on District 1, her ass has gotten firmer.
[CORRECT; this sentence is correct for the same reason that sentence 9 is wrong.]