A hyphen (-) is a punctuation mark that’s used to connect words or parts of words:

A one-night stand with a thirty-five-year-old jack-of-all-trades.
A time of self-reflection about one’s decision-making skills.
A feeling of short- and then long-term regret.

Like most punctuation, using hyphens correctly can be a tricky matter that many people never quite master. And while it would take pages and pages to cover all of the nuances of hyphen usage, we can cover the bases pretty quickly. Here goes:

Use hyphens with compound adjectives that appear before nouns

Any time you stack two or more words together in front of a noun, so that they collectively function as an adjective, you’ve created what’s called a compound adjective. Compound adjectives are generally hyphenated:

He’s looking for a cat-friendly apartment.
She had a very matter-of-fact expression on her face.

The key is that those compound adjectives need to come before the noun they’re modifying. Otherwise you don’t use hyphens:

He’s looking for an apartment that’s cat friendly.
The expression on her face was very matter of fact.

And while you can use multiple words in your compound adjectives (a hard-to-find place, a soon-to-be-forgotten example), you don’t want to overdo it:

This is bad → An around-the-world-all-expenses-paid-once-in-a-lifetime vacation.

Last, but not least, don’t use hyphens with adverbs:

Incorrect: The cleverly-disguised detective.
Correct: A mercifully short example.

Use suspended hyphens to avoid redundancy in a series

Imagine you’re describing a new kind of fabric. You could write:

We’re selling a new stain-resistant, wrinkle-resistant, and odor-resistant fabric.

However, a more concise way to express the same idea is to use suspended hyphens:

We’re selling a new stain-, wrinkle-, and odor-resistant fabric.

In general, don’t use hyphens with prefixes (although there are exceptions)

Resist the temptation to hyphenate any word that starts with a prefix:

Write coauthor, not co-author
Write nonviolent, not non-violent
Write reedit, not re-edit

Having said that there are many exceptions. Here are two rules to help you find the more common ones:

1) use a hyphen before a capitalized word or a numeral (pre-Columbian, post-1970)

2) to separate two i’s, two a’s, and other combinations of letters that could easily be misread, such as anti-inflammatory, mega-annoyance, or pro-life.

Be careful when using hyphens to express age and numbers

In general, hyphenate ages in both their noun and adjective forms:

a nine-year-old
a ten-year-old boy
a thirty-four-year-old mother
a group of eight- to ten-year-olds

but

eighteen years old
seventy years of age

Hyphenate numbers when written out or expressed as a fraction, but not when expressed as percentages:

Q: If twenty-eight people came to the party and two-thirds of them got drunk, what percentage of them woke up with a hangover the next day?

A: 66 percent.

Have we answered all of your hyphen-related questions?

Maybe, maybe not. If you really want to geek out on hyphens, check out this great reference page from the good folks over at The Chicago Manual of Style. It goes really deep. And, if you’re still in doubt about hyphen usage, look it up in the dictionary or, better yet, let Acrolinx help you make sure you’re using them correctly.