Hyphens are used to combine two words into one, as in compound adjectives.

Example: Not hyphenating compound adjectives is an all-too-common error.

Here the words “all,” “too,” and “common” are working together to modify the noun “error” so they need to be hyphenated. Common compound adjectives include ages (two-year-old dog, twelve-year-old child), adverb-as-adjective combos (dimly-lit room, well-behaved cat), and compound word adjectives (full-time job, multi-unit housing).

Sometimes in English, words that are frequently hyphenated become fused (The word “crybaby” used to be hyphenated.) or permanently split (“Ice cream” used to be hyphenated.) depending on how people use them. Some words are on the cusp and are generally acceptable both ways (“preeminent” or “pre-eminent”). Occasionally words need to be hyphenated to prevent confusion.

Example: You should re-cover your boat after you recover from your sprained ankle.