Here are some examples of when hyphens are used:
* Compound adjectives: "well-known" "state-of-the-art"
* Numbers: "twenty-one" "two-thirds"
* Prefixes: "pre-existing" "anti-inflammatory"
* To avoid ambiguity: "re-cover" (to cover again) vs. "recover" (to get better)
There are specific rules for hyphenation, and it's important to consult a style guide or dictionary for specific cases.