There are two commandments about this misunderstood punctuation mark. First, hyphens must never be used interchangeably with dashes (see the “Dashes” section), which are noticeably longer. Second, there should never be spaces around hyphens.
Incorrect: 300—325 people
Incorrect: 300 – 325 people
Correct: 300-325 people
Hyphens’ main purpose is to glue words together. They notify the reader that two or more elements in a sentence are linked. Although there are rules and customs governing hyphens, there are also situations when writers must decide whether to add them for clarity.
Hyphens Between Words
Rule 1. Generally, hyphenate two or more words when they come before a noun they modify and act as a single idea. This is called a compound adjective.
Examples:
an off-campus apartment
state-of-the-art design
When a compound adjective follows a noun, a hyphen may or may not be necessary.
Example: The apartment is off campus.
However, some established compound adjectives are always hyphenated. Double-check with a dictionary or online.
Example: The design is state-of-the-art.
Rule 2a. A hyphen is frequently required when forming original compound verbs for vivid writing, humor, or special situations.
Examples:
The slacker video-gamed his way through life.
Queen Victoria throne-sat for six decades.
Rule 2b. When writing out new, original, or unusual compound nouns, writers should hyphenate whenever doing so avoids confusion.
Examples:
I changed my diet and became a no-meater.
No-meater is too confusing without the hyphen.
The slacker was a video gamer.
Video gamer is clear without a hyphen, although some writers might prefer to hyphenate it.
Writers using familiar compound verbs and nouns should consult a dictionary or look online to decide if these verbs and nouns should be hyphenated.
Rule 3. An often overlooked rule for hyphens: The adverb very and adverbs ending in -ly are not hyphenated.
Incorrect: the very-elegant watch
Incorrect: the finely-tuned watch
This rule applies only to adverbs. The following two sentences are correct because the -lywords are adjectives rather than adverbs:
Correct: the friendly-looking dog
Correct: a family-owned cafe
Rule 4. Hyphens are often used to tell the ages of people and things. A handy rule, whether writing about years, months, or any other period of time, is to use hyphens unless the period of time (years, months, weeks, days) is written in plural form:
With hyphens:
We have a two-year-old child.
We have a two-year-old.
No hyphens: The child is two years old. (Because years is plural.)
Exception: The child is one year old. (Or day, week, month, etc.)
Note that when hyphens are involved in expressing ages, two hyphens are required. Many writers forget the second hyphen:
Incorrect: We have a two-year old child.
Without the second hyphen, the sentence is about an “old child.”
Rule 5. Never hesitate to add a hyphen if it solves a possible problem. Following are two examples of well-advised hyphens:
Confusing: I have a few more important things to do.
With hyphen: I have a few more-important things to do.
Without the hyphen, it’s impossible to tell whether the sentence is about a few things that aremore important or a few more things that are all equally important.
Confusing: He returned the stolen vehicle report.
With hyphen: He returned the stolen-vehicle report.
With no hyphen, we could only guess: Was the vehicle report stolen, or was it a report on stolen vehicles?
Rule 6. When using numbers, hyphenate spans or estimates of time, distance, or other quantities. Remember not to use spaces around hyphens.
Examples:
3:15-3:45 p.m.
1999-2016
300-325 people
Rule 7. Hyphenate all compound numbers from twenty-one through ninety-nine.
Examples:
thirty-two children
one thousand two hundred twenty-one dollars
Rule 8. Hyphenate all spelled-out fractions.
Example: more than two-thirds of registered voters
Rule 9. Hyphenate most double last names.
Example: Sir Winthrop Heinz-Eakins will attend.
Rule 10. As important as hyphens are to clear writing, they can become an annoyance if overused. Avoid adding hyphens when the meaning is clear. Many phrases are so familiar (e.g.,high school, twentieth century, one hundred percent) that they can go before a noun without risk of confusing the reader.
Examples:
a high school senior
a twentieth century throwback
one hundred percent correct
Rule 11. When in doubt, look it up. Some familiar phrases may require hyphens. For instance, is a book up to date or up-to-date? Don’t guess; have a dictionary close by, or look it up online.
Hyphens with Prefixes and Suffixes
A prefix (a-, un-, de-, ab-, sub-, post-, anti-, etc.) is a letter or set of letters placed before a rootword. The word prefix itself contains the prefix pre-. Prefixes expand or change a word’s meaning, sometimes radically: the prefixes a-, un-, and dis-, for example, change words into their opposites (e.g., political, apolitical; friendly, unfriendly; honor, dishonor).
Rule 1. Hyphenate prefixes when they come before proper nouns or proper adjectives.
Examples:
trans-American
mid-July
Rule 2. For clarity, many writers hyphenate prefixes ending in a vowel when the root word begins with the same letter.
Example:
ultra-ambitious
semi-invalid
re-elect
Rule 3. Hyphenate all words beginning with the prefixes self-, ex- (i.e., former), and all-.
Examples:
self-assured
ex-mayor
all-knowing
Rule 4. Use a hyphen with the prefix re- when omitting the hyphen would cause confusion with another word.
Examples:
Will she recover from her illness?
I have re-covered the sofa twice.
Omitting the hyphen would cause confusion with recover.
I must re-press the shirt.
Omitting the hyphen would cause confusion with repress.
The stamps have been reissued.
A hyphen after re- is not needed because there is no confusion with another word.
Rule 5. Writers often hyphenate prefixes when they feel a word might be distracting or confusing without the hyphen.
Examples:
de-ice
With no hyphen we get deice, which might stump readers.
co-worker
With no hyphen we get coworker, which could be distracting because it starts with cow.
A suffix (-y, -er, -ism, -able, etc.) is a letter or set of letters that follows a root word. Suffixes form new words or alter the original word to perform a different task. For example, the noun scandal can be made into the adjective scandalous by adding the suffix -ous. It becomes the verb scandalize by adding the suffix -ize.
Rule 1. Suffixes are not usually hyphenated. Some exceptions: -style, -elect, -free, -based.
Examples:
Modernist-style paintings
Mayor-elect Smith
sugar-free soda
oil-based sludge
Rule 2. For clarity, writers often hyphenate when the last letter in the root word is the same as the first letter in the suffix.
Examples:
graffiti-ism
wiretap-proof
Rule 3. Use discretion—and sometimes a dictionary—before deciding to place a hyphen before a suffix. But do not hesitate to hyphenate a rare usage if it avoids confusion.
Examples:
the annual dance-athon
an eel-esque sea creature
Although the preceding hyphens help clarify unusual terms, they are optional and might not be every writer’s choice. Still, many readers would scratch their heads for a moment overdanceathon and eelesque.
Source: http://www.grammarbook.com/punctuation/hyphens.asp
