Language is one of those things that come naturally to human beings. People are born without the ability to talk, but as they hang out with people who cantalk, the vast majority pick it right up. I know people who could talk all day--often in complete sentences--having never spent a day in school. Some of these people aren't even potty trained yet.
And, by the way, even if you think of yourself as being bad at grammar, it isn't true. You get your grammar correct almost all the time. For instance, you have never accidentally placed an adjective after the noun it modifies. That is to say, you have never said "llama disgruntled" when you meant to say "disgruntled llama." And even if you can't identify an adjective clause, you have never made the mistake of putting one before the noun it modifies. You have never said "the who teaches my karate class retired librarian" when you meant to say "the retired librarian who teaches my karate class."
But if spoken language comes naturally and organically to human beings, writing doesn't. Punctuation certainly doesn't. It's something you have to learn. And, unfortunately, writing is not merely a matter of putting down on paper the words that you would utter if you were speaking. Spoken language is incredibly rich and layered--more rich and more layered than can be expressed in writing. Besides the words themselves, the speaker has at his or her disposal a number of tools: intonation, facial expressions, hand gestures, pacing, and other tools that I am no doubt forgetting. Furthermore, the hearer's capacity for interpreting all those signals is equally astonishing.
Writing, by comparison, is pretty flat. I find that about half of writing instruction is helping people make their writing feel less flat and more like spoken language. But the other half of writing instruction is making sure that writers stay within the comparatively narrow confines of intelligibility. By "narrow confines of intelligibility" I simply mean that the writer doesn't have those extra-verbal tools to work with; all you've got is letters, spaces, and punctuation marks.
Like street signs, punctuation marks are a wholly arbitrary and wholly necessary. On the one hand, they simplify and clarify language and keep it within bounds that allow the reader to make sense of things, in spite of the fact that the reader is at a considerable handicap compared to the listener. At the same time, punctuation adds information that gets lost in the translation from spoken language to written language. A speaker doesn't insert a question mark at the end of a question; he doesn't have to, because his intonation will serve. But a writer had better include a question mark at the end of every question and a period at the end of every declarative, or the reader will get lost in a hurry.
This is supposed to be a letter about dashes, so let's move along. I like dashes. Dashes bring back to written prose some of the flexibility that other punctuation marks take away by necessity. Or, at the very least, dashes give an impression of flexibility that can make written prose feel a little more like spoken language.
The dash is the most interrupt-y of punctuation marks. It is useful for inserting extra information--sometimes expressed in less-than-precise grammar--into a sentence, in much the same way that a speaker frequently interrupts himself mid-stream as new ideas occur to him.
In the last couple of weeks I have laid down some relatively strict rules for the use of semicolons and colons. The rules for dashes aren't quite so strict. Perhaps the strictest rule is simply that you shouldn't annoy your reader by over-using the dash.
Here are some common uses of the dash:
Use the dash to set off "bonus" information. I used a pair of dashes a paragraph or two above:
It is useful for inserting extra information--sometimes expressed in less-than-precise grammar--into a sentence.
That phrase, "sometimes expressed in less-than-precise grammar" is a little bonus aside that I added to the sentence. I might have used parentheses, but parentheses in prose often feel to me like an invitation to ignore whatever information is inside them. A dash signals that this information is extra, but not exactly parenthetical. Why didn't I set the phrase off with commas? As I said earlier, the dashes signal that this is bonus information; it doesn't quite rate a pair of commas.
Use the dash to set off longish and/or surprising appositives. Like many uses of the dash, this one is a judgment call or a matter of style. For most appositives, a comma (or pair of commas) will serve, but a dash (or pair of dashes) draws a little extra attention to the appositive as something worthy of interest.
John Barber--my long-time nemesis and bad influence--is coming to town this weekend.
As you can see, the dashes have the added benefit of helping the reader keep track of the relationship between the subject (John Barber) and the verb (is coming), even though nine words intervene between them.
On a related note, you should always use dashes to set off an appositive when there are commas in the appositive.
The Three Stooges--Larry, Moe, and Curly--entered with much slapping and nose-tweaking.
This is one of the few situations in which the dash is not a matter of preference, but the only appropriate punctuation mark.
Use a dash to set off surprising information. As in the following example:
I'm so hungry I could eat a horse--literally.
You could also use a period between "horse" and "literally," but I prefer the dash, since a period would introduce a sentence fragment.
If you want to read more about dashes, I found this Grammar Girl article pretty helpful.
Also, let me say that a dash is NOT the same thing as a hyphen. A dash is made up of two hyphens and does not have spaces before or after. (Most word-processing programs will turn two hyphens into a dash, but as you may have noticed, Mail Chimp does not). Here's a very good accounting of the uses of the hyphen, in case your hunger for information about horizontal punctuation is not yet sated.
You're liable to get conflicting information regarding the uses of the dash. I suppose this is unavoidable when we are talking about a punctuation mark whose whole raison d'être is to introduce flexibility into sentences. But I would summarize the benefits of the dash by returning to a point I made earlier in this letter: written language, more or less by definition, is a little flat and a little rigid compared to the spoken language that comes naturally to us. The dash can enliven prose by allowing a writer a little more of the flexibility that we expect in spoken language.
I will conclude with an example that illustrates what I mean. Here are a few facts that we are going to combine into a single sentence:
I had an uncle named Bill.
Bill came from Cuba.
Bill married my Aunt Aline.
Bill carried Aline to Washington State.
English grammar gives us a number of options for putting this information into a sentence. Here are a couple:
My uncle Bill, who came from Cuba, married Aline and carried her off to Washington State.
My Cuban Uncle Bill married Aline and carried her off to Washington State.
My Uncle Bill--the one who came from Cuba--married Aline and carried her off to Washington State.
There's nothing wrong with Sentence 1 or 2. Both convey all the necessary information. But can you feel how Sentence 3 just feels a little more like a sentence that a person might utter in the world God made? The grammar in Sentence 3 is a little odd. But that's why it feels more conversational. The flexibility of the dash allows you to introduce that strange, interrupt-y, but authentic grammar.