As a general rule, two or more words that act together to modify another word are hyphenated, provided that they appear before the word they modify:
end-of-the-month report well-known researcherbuta report filed at the end of the month researcher that was well known
Compound numbers from twenty-one to ninety-nine are hyphenated, together with compound adjectives that begin with a numeral or number:
sixty-one twenty-seven 16-unit division seven-dollar seats
Two or more words that act together to form a new meaning are hyphenated:
bald-faced good-looking double-check self-satisfied
Hyphens are used to avoid a double-vowel or triple-consonant string:
not but
antiintellectual anti-intellectual Falllike weather Fall-like weather crossstich cross-stitch
Connected words that act like adjectives are not hyphenated if the first word ends in -ly:
clearly defined roles highly structured exercise partially completed project poorly conceived design
Prefixes are generally not hyphenated, unless they precede a capitalized word. However, some prefixes ending in vowels use hyphens as connectors:
midpoint undersea mid-December un-American