Hyphens have always confused me, but since I have begun to edit my work and the work of others, I have had to learn how they are used. The use of hyphens involves certain rules, none of which are that hard to remember.

Not everyone agrees on when a hyphen should be used, but there are some definite rules that everyone does agree upon. Let's examine the rules for using a hyphen.

A hyphen should be used to join two or more words that serve as a single adjective in front of a noun.
Ben gave his girlfriend a box of chocolate-covered cherries for Valentine's Day.
The reddish-brown chair is the one Dad decided he was going to purchase.
But if the adjectives come after the noun, they are not hyphenated.
The cherries were chocolate covered.
The chair is reddish brown.

When a word begins with the prefixes all, ex, and self a hyphen is required.
Stephen was thinking about buying an all-terrain vehicle.
Donna's ex-husband was stalking her.
Please exhibit some self-control.

All prefixes that are followed by a proper noun must use a hyphen, as must all prefixes that are followed by numbers.
The document was found to be from the pre-Columbian era.
The marriage took place during the mid-1980s.

Hyphens are used with all compound numbers.
She will turn twenty-six on October 12 of this year.
Thirty-five women belong to the knitting club.

Sometime it is necessary to use a hyphen in order to avoid confusion or an awkward combination of letters.
It will be necessary to re-sign the agreement.
Marcia was half-frozen when she came back inside after playing in the snow and ice.

Hyphens are used to divide words at the end of a line, however, you need to be sure and divide them between syllables.

When written as adjectives, fractions are hyphenated. When written as nouns, they are not hyphenated.
A five-eighths screw is required.
Two thirds of its capacity has been filled.

There are no hyphens in words that begin with the following prefixes: inter, iso, mis, over, poly, pseudo, psycho, sub, supra, trans, and un. There are absolutely no exceptions to this rule.

Words that begin with the following prefixes normally don't take hyphens, but there are exceptions with each one of them. I found this information in How To Say It and Write It Correctly Now, which was written by Dr. Santo J.Aurelio. This guy has a doctorate in education from Boston University and has mainly taught English grammar, so I'm inclined to believe he is correct in what he says.

mono - The one exception is mono-ion.
multi - The exceptions here are multi-ply and all multi-i words, such as multi-industry.
non - The two exceptions are non-oil and non-euclidean.
out - The four exceptions are out-front, out-group, out-Herod, and out-migrate.
post - The two exceptions are post-horse and post-obit.
pre - The exception is pre-engineered.
retro - The two exceptions are retro-engine and retro-rocket.
semi - The exceptions are semi-antique, semi-double, and all semi-i words, such as semi-independent.
super - The one exception is super-duper.
ultra - The one exception is ultra-abyssal.
up - The five exceptions are up-anchor, up-bow, up-country, up-tempo, and up-twister.

Always remember, if you are in doubt about whether or not to hyphenate a word, you can always consult your dictionary.