The dash separates; the hyphen (-) joins. Its main job is to show that two or more words work together as one describing word in front of a noun: a well-known author.
Key idea
Hyphenate a compound modifier before a noun (a two-story house); usually not after it (the house is two stories tall). Never hyphenate after an -ly adverb (a brightly lit room). Hyphenate the numbers twenty-one to ninety-nine when you write them as words.
Why it works
A hyphen can change the meaning. A small-animal doctor treats small animals; a small animal doctor could be a doctor who is small. A man-eating shark is dangerous; a man eating shark is a man having lunch.
Worked example: Before and after the noun
“a ten year old boy” and “The boy is ten years old.”
- In the first, ten year old works as one adjective before boy: hyphens.
- In the second, the words come after the noun as a phrase: no hyphens.
Answer: a ten-year-old boy; The boy is ten years old.
Worked example: Numbers
Write 36 and 125 in words.
- Thirty-six is between twenty-one and ninety-nine: hyphen.
- In 125, only the twenty-five part gets one.
Answer: thirty-six; one hundred twenty-five
Common mistake
Hyphenating after an -ly adverb (“a newly-built house”). The -ly already tells the reader that the word describes the next one, so style guides say no hyphen. Some words have hyphens of their own (self-aware, ex-coach, all-star); when in doubt, check a dictionary.