A reader asked me to explain when to use a comma, a semi-colon and a period. It’s like trying to explain how you get somewhere you’ve driven a thousand times. It’s so second nature for me, it’s hard to explain it. But let me try.
A comma marks a short pause. It helps clarify when to take a quick break in a sentence. (Which makes it easier for the reader to understand the thought, and can affect meaning.) Here’s a classic example:
"Let’s eat, grandma." makes it clear the kids want grandma to know they’re hungry.
"Let’s eat grandma." (with no comma) turns grandma from the audience into the main course. (A great take-that to people who think commas are optional.)
A semi-colon is more of a throat-clearing; it distinguishes two separate thoughts that are related. “The kids were eying the leftover chicken; grandma was reaching for the Chardonnay.”
A period is a complete stop. “The kids were starving. Moments later, the pizza guy arrived.” You could argue that these sentences are related and could take a semi-colon. But they’re different enough that a period makes more sense.
The dash — it’s a what-the-hell,-I’m-not-sure-what-to-use mark. It’s made by two hyphens joined together (—) that act like a punctuation Bandaid. You can slap it on a thought when there’s some kind of break, but you’re not sure what punctuation to use.
And let’s not forget the colon: the two dots, the list-setter-upper, the drum roll before a next thought. It’s like a trumpet that announces a forthcoming list or example. It has some other uses, but if we save it for this, life would be easier for all of us. And as punctuation goes, we could all use a break.