The hyphen, or "-," is used to separate syllables at the line breaks of sentences or to separate words, particularly those used in phrases called compound modifiers. Before the advent of word processing, writers with flush-right margins commonly employed the hyphen to divide longer words at the end of each line of text. In 2011, it is a symbol typically used to add prefixes and suffixes, or to separate words used in concert as modifying adjectives, as in "long-winded writers."
The dictionary is useful when determining which words or phrases take a hyphen, and which do not. "Predawn" and "aftereffect," for instance, are established enough not to need the symbol, though prefixes and suffixes generally take them, like with "ex-wife" and "president-elect." Words like "anti-hunting" and "ill-informed" benefit from the hyphen's organizational role.
A hyphen is usually preferred if a word other than the first in a phrase is capitalized, as in "anti-Semitic," or if the juxtaposition is confusing or hard to read without it, like with "co-op" or "anti-immigration." Sometimes, more than one meaning can be inferred from words with the same letters. This is another case when a hyphen becomes necessary. If a person "resigns," that means something completely different than if he or she "re-signs." Also, if words appear at line breaks, they are generally divided at the end of an accentuating syllable, though rules vary on this.
The hyphen is most often seen in modern texts when two words are used as a single idea to describe a noun or pronoun. The "blue-gray sky" or the "often-ill-suited academic" link two or more words as a single adjective. Exceptions do exist, though. For instance, the Associated Press style guide urges writers to resist using the hyphen after words ending in "-ly," as in the "absurdly tardy student." Also, when the compound modifier comes after the noun, the symbol is not needed, such as in "The book's themes were well known."
Other uses for hyphens are commonly used, like when notating age, such as "7-years-old" or when writing out large numbers, such as "eighty-nine." Some adverbs and even a few nouns contain hyphens, such as "on-the-fly" or "son-in-law," respectively. Hyphens are also used when a letter is joined with a word, such as "T-shirt" and "S-curve." Again, the use of a dictionary will illustrate the exceptions, which tend toward removing the symbol after a certain amount of accepted use, as long as the removal does not create "undue" confusion.