Considering that the rules for quotation marks are relatively simple (I mean, compared to something really complicated like the comma), it’s surprising how often we see errors with them.
In dialog, stuff that’s said aloud goes in quotation marks. “I can’t believe she said that.” (Stuff that’s not said aloud is sometimes set in italics.) Simple enough. Few writers struggle with that.
Single quotes are used when you have a quote within a quote: “I can’t believe she said ‘irregardless.’” There are a few other publication styles that call for single quotes—for example, a lot of newspapers use them in headlines because they take up less space—but a quote within a quote is about the only time most writers need to use single quotation marks.
Many writers use single quotes to draw attention to a word that’s being used as a word. According to The Chicago Manual of Style, this practice comes to us from the field of philosophy. But Chicago’s recommendation is to use italics instead.
Sometimes we use double quotes around a term that’s new, or being used in a new way. When green first came to be used to mean environmentally friendly, journalists often put it in quotes. That’s no longer necessary, but the next time a neologism comes along, feel free to break out those quotes the first time you explain it. But after that, retire them.
The use of scare quotes, or putting something in “quotation marks” to draw attention to it, is discouraged, as it implies irony, or that the marked word or phrase isn’t what it pretends to be. It’s the typographical equivalent of making air quotes with your fingers. Or the other way around. The shopkeeper whose sign advertises “Fresh” Produce is implying that his produce isn’t really fresh, it’s just pretending to be. So be extra careful with those scare quotes. Use them too often, and you’ll scare off readers.