“The basic rule of hyphens . . . is that they’re used to form compound modifiers, that is, to link two or more words that are acting as adjectives or sometimes adverbs.” (Casagrande, 2006, p. 62)
Okay, what? What are compound modifiers?
The rule is actually simpler than it seems. Essentially, when you have two or more words that modify or describe a noun that follows, you should hyphenate those words. But a word of caution: you shouldn’t hyphenate the same words if they come after the noun.
I thought we were in a long-term relationship.
Everyone knew that relationship was not long term.
I really need a fuel-efficient car to save money for more video games.
To save money for video games, I bought a car with better fuel efficiency.
I have a three-year-old son who mimics every word you say.
I have a son who mimics every word you say; he is three years old.
Hyphens also have other uses including acting with prefixes, suffixes, nouns, letters, and numbers, and clarifying the meaning of words. Generally, you will hyphenate words that begin with self, all, ex, and words that begin with a capital letter or number. Here are some examples:
Of course, we should also hyphenate compound numbers like twenty-five or thirty-seven.
Finally, it’s important to note that hyphen “rules” are more like the Pirate Code in that they are really more like guidelines. Even the “experts” will disagree about whether or not some words or groups of words should be hyphenated. It’s definitely a good idea to double check with the style manual you are using, such as the APA or MLA manuals, and for tricky words, you can consult a good dictionary.