• Join two or more words used as one adjective or adverb:
  • a tried-and-true eggbeater • working after-hours
  • a two-page letter
• Join two or more words used as one verb:
  • to fine-tune • to double-click • to dry-clean •to second-guess
• Join two or more words used as one noun
  • father-in-law • go-between • runner-up • time-out • two-by-four
• Separate units in serialized numbers such as phone number, invoices, zip codes, SS #'s, etc:
  • model G-01234 • invoice # ABC-12345 • 630-697-2501
• Express numbers as words:
  • twenty-one • twenty-one hundred • one twenty-one hundredth
• Express fractions as words:
  • one-quarter of a pound [BUT: a quarter pound of butter]
  • one-half hour [BUT: a half hour, half an hour]
• To set off prefixes or suffixes:
  • Actually, this should generally not be done [overconfident, coauthor, reorganize, subdivision, nationwide, pseudoscientific, midstream, nonindustrial]. However, there are exceptions:
  • a hyphen normally follows “mid” in expressions involving numbers
  • if three ll’s occur in succession, a hyphen should be used
  [shell-less, bell-like].
  • when truly necessary, a hyphen should be used to prevent one word
  from being mistaken for another [lock the coop / buy a co-op, multiply
  by 12 / a multi-ply fabric, a unionized factory / an un-ionized substance].
  • as a rule, when adding a prefix to a hyphenated or spaced
  compound word, use a hyphen after the prefix [pre-high school years,
  post-bread-winning years, ex-attorney general, non-interest-bearing
  notes, non-civil service position].
  • when the prefix ends with “a” or “i” and the base word begins with the
  same letter, use a hyphen to prevent misreading: [ultra-attractive,
  anti-intellectual, semi-independent]. (Note: just the opposite is true
  for “e” and “o”: coordinate, preeminent, cooperate, reelect).
  • use a hyphen after “self” when it serves as a prefix, but not when
  it’s actually the base word itself, followed by a suffix: [self-confidence,
  self-destruct, self-help (BUT: selfish, selfless, selfsame)]

The en dash is created by using the Option-shift-hyphen

• Use in place of the word “to” in order to link two figures that represent a continuous sequence:
  • On pages 12–14 • during the week of March 22–28 • 1921–1927

• Use to set off a phrase within a sentance:
  • Many of the components—for example, the motor—are manufactured here.
  
• Use as an alternative to other punctuation, such as comma, colon, semicolon, or even parenthesis—especially if you’ve used others already or you want stronger emphasis.
  • I do the work—he gets the credit!