A hyphen joins two or more words together while a dash separates words into parenthetical statements. The two are sometimes confused because they look so similar, but their usage is different. Hyphens are not separated by spaces, while a dash has a space on either side.
Generally, hyphens are used to join two words or parts of words together while avoiding confusion or ambiguity. Consult your dictionary if you are not sure if a hyphen is required in a compound word, but remember that current usage may have shifted since your dictionary was published.
There are some cases where hyphens preserve written clarity such as where there are letter collisions, where a prefix is added, or in family relations. Many words that have been hyphenated in the past have since dropped the hyphen and become a single word (email, nowadays).
In some cases though, a hyphen does change the meaning of a sentence.
- I am thinking of re-covering my sofa (= to put a new cover on it)
- I would like to recover my sofa. (= from someone who has borrowed or stolen it)
Hyphens in numbers
Use a hyphen with compound numbers from twenty-one to ninety-nine.
In written fractions place a hyphen between the numerator and denominator except if there is already a hyphen in either the numerator or the denominator.
- sixty-nine eighty-ninths
Use a hyphen when a number forms part of an adjectival compound
- France has a 35-hour working week.
- He won the 100-metre sprint.
- Charles Dickens was a great nineteenth-century novelist.
Dashes can be used to add parenthetical statements or comments in much the same way as you would use brackets. In formal writing you should use the bracket rather than the dash as a dash is considered less formal. Dashes can be used to create emphasis in a sentence.
- You may think she is a liar - she isn't.
- She might come to the party - you never know.