A hyphen is the sign (-) used to join words to indicate that they have a combined meaning or that they are linked in the grammar of a sentence (as in pick-me-up, rock-forming), to indicate the division of a word at the end of a line, or to indicate a missing or implied element (as in short- and long-term).
Hyphens help clarify meaning. For example a light purple bag is different from a light-purple bag. Without the hyphen you are saying that you have a bag that is both purple and light. With the hyphen you are saying that your bag is light-purple in color.
Or let's say you've got twenty two year olds. Without a hyphen, it is not clear if you mean that you run a day care and have twenty two-year-olds, or if you have twenty-two year olds applying for the job of nanny.
Hyphen errors simply come from putting then in the wrong spot or omitting them altogether. Here are a few quick tips:
1. For adjectives, the trick is to make sure that words you've got linked by the hyphen are being used as one word to modify the noun. When both words seperately modify the same noun, they are not hyphenated.
2. For nouns (and as a second check on all hyphens) a good dictionary is your best bet.
3. Do not use spaces around hyphens unless you have a list where one hyphenated item is being split: The pre- and post-game specials are a waste of time.
Source: Dictionary Version 2.1.2 (80.3) Copyright © 2005–2009 Apple Inc. and http://www.flickr.com/photos/hayzeleyes/4954905220/
Hyphenate simple fractions:
one-third
two-fifths
seven-eighths
Hyphenate compound numbers from 21 through 99:
twenty-one
ninety-nine
thirty
one thousand six hundred seventy-eight
Source: Made by me.
ADJECTIVES
A compound adjective is hyphenated if the hyphen helps the reader differentiate a compound adjective from two adjacent adjectives that each independently modifies the noun. Compare the following examples:
Hyphenate two or more words acting as a single adjective before a noun, but DO NOT hyphenate of they come after the noun.
Hyphenate an adjective preceding a noun to which -d or -ed has been addedand it is used before a noun:
Hyphenate compound adjectives when consisting of a noun, adjective, or adverb preceding an -ing word:
Hyphenate compound adjectives that are colors
Hyphenate compound adjectives with high- or low-:
Hyphenate compound adjectives that are comparatives or superlatives in compound adjectives (unless you are using more or most, less or least).
Hyphenate compound adjectives for effect
NOUNS
Compound nouns come in three types
Usage often depends on where you are and the choice of the writer, so honestly a good dictionary is your best bet, but here are a few tips.
Hyphenate compound nouns that contain suffixes like -er, -ed, -ness
home-wrecker
Hyphenate compund nouns that contain articles, prepositions or conjunctions
Source: Made by me with help from http://www.flickr.com/photos/smith/15620160/, http://www.flickr.com/photos/housingworksauctions/509428714/, and http://en.wikipedia.org/wiki/English_compound