If you think placing commas rightly is hard enough, try reading this.
1. Omitting Hyphens in Phrasal Adjectives
Some phrasal adjectives (including “civil rights,” “stock market,” and “high school”) don’t require hyphenation when they appear before a noun; they’re so well entrenched in the language that no risk of ambiguity exists, and their status is enshrined by inclusion in dictionaries.
But when two words team together to describe a noun, they’re usually hyphenated. (Leave them open after a noun, however.) If you can’t find them in your well-thumbed dictionary, attach them — and don’t hesitate to link more than two words: “The company instituted a pay-as-you-go plan.”
2. Adding Hyphens to Compound Words
Compound words come in three forms: open (“sand dollar”), hyphenated (sand-blind), and closed (sandbag). As you see from these examples, compounds including the same particular word are not necessarily treated the same; compounding is a random process related to usage. (Popular treatment of long-hyphenated compounds changes so rapidly that dictionaries change them in new editions to reflect prevailing usage; pigeonhole — formerly pigeon-hole — is just one example.)
3. Adding Hyphens to Prefixes
Prefixes, on the other hand, are almost always closed up to the root word. Exceptions include when the root word is a proper name (pre-Christian) and when the prefix ends and the root word begins with an i (anti-inflammatory). Note, however, that this is not true in the case of e (preempt). Another exception is words beginning with c preceded by co-, because to many people, terms like co-chair look awkward without a hyphen.
4. Omitting Hyphens from Potential Homographs
Sometimes, prefixed words that would otherwise be closed up retain a hyphen to distinguish them from otherwise identical-looking words, such as re-cover as opposed to recover and re-creation as distinct from recreation.
5. Omitting Hyphens in Verb Phrases
Compound verbs, those consisting of more than one word, are hyphenated (test-drive) or closed (troubleshoot); the dictionary will let you know which form to employ. Note, however, the difference in nearly identical-looking compound verbs and open compound nouns: “I’m going to test-drive it tomorrow,” but “I’m going to take it on a test drive tomorrow.”
Also, consider the subtle difference between gerunds formed from a hyphenated compound verb that are followed, or not followed, by an object: “I was spot-checking the report when I found a serious error,” but “I’m going to do a little spot checking.”
6. Adding Hyphens to Adverbial Phrases
Adverbs are not attached to adjectives when they team up to modify a noun: “The slowly melting ice rendered the river crossing a perilous enterprise.” However, the presence of an adverb does not negate the need for a hyphen in a phrasal adjective that follows it: “Hers was an eloquently sharp-tongued response.”
7. Adding Hyphens to Prepositional Phrases
Phrases telling the reader to do something in which the first word is a verb and the second is a preposition are not hyphenated: “Sign in at the registration table.” (The phrase is hyphenated, however, when it modifies a noun: “Go to the sign-in table.”)
8. Adding or Omitting Hyphens When Referring to Ages or Physical Dimensions
When a person is identified by their age with the phrase “seven-year-old,” for example, the phrase is hyphenated whether it modifies child, boy, girl, and so on or the noun is implied. (Note that two hyphens are necessary and that, for the spelled-out form of a two-digit number, three are required: “twenty-seven-year-old.”) However, the constituent words are unattached when the phrase follows the noun: “The child is seven years old.
By the same rules, words describing an object’s physical dimensions are similarly linked: “Cut the eight-foot-long board in half.” Note, again, that all the words describing the length of the board are attached: If the final hyphen is incorrectly omitted, the reference to a board that is eight feet long is erroneously changed to describe a long board with eight feet.
9. Omitting Letter Spaces When Using Hyphens
When you see a hyphen followed by a letter space, don’t assume the space is an error. “The assignment is a 2,000- to 5,000-word essay” is correct; word has been omitted after the first number because it is implied by its presence after the second number. (This usage is called suspensive hyphenation.)
10. Confusing Hyphens and Dashes
Many publications, for the sake of simplicity or because the producers don’t know any better, use single hyphens in place of em dashes or double hyphens (the less aesthetically pleasing alternative that is frequently employed online). But they look stubby and ugly, and this crime against aesthetics is compounded when letter spaces around them are omitted, producing abominations such as “The key-and this is important-is to keep stirring constantly.”