Here are some examples of when hyphens are used:
* Compound adjectives: "well-known" "state-of-the-art" "two-year-old"
* Numbers: "twenty-one" "one-third"
* Prefixes: "pre-existing" "anti-inflammatory"
* Fractions: "one-half" "two-thirds"
Important Note: There are many rules and exceptions for hyphenation. It's helpful to consult a dictionary or style guide for specific word combinations.