For most writers, the hyphen’s primary function is the formation of certain compound terms. The hyphen is also used for word division, which is briefly explained here.
Never use a hyphen in place of an en dash or an em dash.
Compound terms are those that consist of more than one word but represent a single item or idea. They come in three styles.
Open (or spaced) compounds are written as separate words.
chief of staff
Hyphenated compounds use hyphens between the words.
Closed (or solid) compounds are written as a single word.
Compound nouns are the easiest to deal with: most of them can be looked up in a good dictionary. Keep in mind, though, that many compound nouns start out spaced or hyphenated before eventually becoming solid, with dictionaries often lagging behind current usage.
Compound verbs (e.g., waterproof, highlight, rubber-stamp, nickel-and-dime) also are typically included in a dictionary.
The most difficult compound terms to deal with are the compound adjectives. For one thing, most of them will not be found in any dictionary. For another, whether they are hyphenated or not depends on their position within a sentence. Whether to hyphenate or not is often a matter of style. Some basic guidance is offered below.
Two or more words that collectively act as an adjective should be hyphenated when they appear immediately before the noun they modify. This helps prevent misreading.
Voters are fed up with this do-nothing congress.
The victim is being described only as a twenty-five-year-old man.
Does this come with a money-back guarantee?
The house comes with a state-of-the-art security system.
Though the one-bedroom apartments are sold out, we still have several two-, three-, and four-bedroom units available.
The major exception is when the compound adjective begins with an adverb ending in -ly. In that case, since a misreading is unlikely, the hyphen is unnecessary. If the -ly adverb is part of a larger compound adjective, use a hyphen.
This is a poorly produced movie.
He followed up with a not-so-poorly-produced sequel.
Forming compounds with an en dash
Certain particularly complex compounds can be formed with an en dash rather than a hyphen, as explained here.
In professionally printed material (particularly books, magazines, and newspapers), the hyphen is used to divide words between the end of one line and the beginning of the next. This allows for an evenly aligned right margin without highly variable (and distracting) word spacing. The rules for such word division are beyond the scope of this guide; they are also beyond the needs of most writers. If you are writing for a publication that requires it, the word division will be handled by the typesetter. Your word processor’s default setting, which is to avoid word division at the end of each line, is appropriate for nearly all of your writing—academic, business, or personal.