Dear QWT: How do you know when to use commas, hyphens or semicolons?
This is an exceptional question without a definitive answer. However, there are a few general rules of thumb you can rely on to help you decide which punctuation to use where.
Commas are used in four general instances:
a) With conjunctions, which are two-part sentences connected with and, but and or:
We walked to the store, but it was closed when we got there.
b) To break out lists of items:
Tom, Dick and Harry were all interested in learning about polyamory.
Note that you do not use a comma before the and.
c) To set off a parenthetical idea or phrase:
Mario Vargas Llosa, the author of Aunt Julia and the Scriptwriter, won the Nobel Prize for Literature in 2010.
d) To delineate a prepositional phrase:
While walking down the street, he was accosted by a group of nymphomaniacs.
The correct term for this punctuation is a dash (hyphens are used to connect words in phrases such as he was in a semi-conscious state). The easiest way to think of a dash is to consider it a comma on steroids. Use it to emphatically set off a parenthetical or prepositional phrase.
One piece of advice on dashes--don't overuse them or they will lose their impact.
Long-time readers of Quick Writing Tips know never to use semicolons because they enable complicated, confused (and quite often bad) writing. If I had the power, I'd permanently ban all semicolons.
But if you must use a semicolon, please use it correctly. That is, use it between two complete and closely-related sentences:
It's almost midnight; we can't make it to the bar before last call.