Hyphens are used in compound adjectives, which are two or more words that make an adjective. Hyphens are also used for some prefixes, numbers and words with multiple letters in a row.
Hyphens are used often in compound adjectives. A compound adjective is when two or more words are combined to make an adjective, for example, "long-term relationship" or "fire-proof vest." In these instances, the compound adjective comes before the noun and is a compound modifier in need of a hyphen. If the compound adjective comes after the noun, the hyphen is not needed. For example, "the blanket is fire proof."
Some prefixes, such as "re," "ex" and "mid," require a hyphen. Examples include "ex-girlfriend," "mid-fielder" and "re-read." When the word after a prefix requires a capital letter, a hyphen should separate the prefix and the word. An example is "anti-American."
Hyphens are also used for ages and numbers. For example, hyphens are used when writing "the 3-year-old liked the toy." When written out, the numbers 29 through 99 require a hyphen, for example, "twenty-nine."
Hyphens split up words with the same three letters in a row, for example, "fall-like." Hyphens are also used when joining letters and words such as "X-ray machine" or "A-frame ladder."