Use to avoid ambiguity. She re-covered the hole. vs. He recovered from the fall. Use to avoid duplicated vowels or triple consonants: anti-inflammatory, shell-like. Use to create two-thought compounds: socio-economic.
–and a compound modifier:
Use to link all the words (except the adverb “very” and all adverbs ending in “-ly”) preceding a noun: a full-time job, a first-period goal, a very good grade, an easily remembered concept. When using a string of modifiers before a noun, put the modifier in quote marks instead of using hyphens, for clarity: He won the “Best Roommate in East Hall” award at reunion.
–in suspensive form:
Suspensive hyphenation takes this form: a 10- to 20-year study; but: a 3-percent to 5-percent chance, a $5 million to $6 million project.
Unless the dictionary makes an exception, do not hyphenate: Decision making takes place on many levels; Fundraising is fun.
Use to separate numerals in odds (he has a 5-1 chance), ratios (the student- teacher ratio is 11-1, she won 3-2), fractions that are spelled out (three-fourths). When large numbers are spelled out as in the beginning of a sentence, use to connect a word ending in “-y” to the next word: Fifty-five (but: three hundred).
–and compound proper nouns:
Use to designate dual heritage: Italian-American; Australian-Czech; African-American. Note that Native American, French Canadian, and Latin American are not hyphenated.