Although a hyphen and a dash may appear to be the same at first glance, they are two very different marks of punctuation. Their form is as different as their meaning, a dash being twice as long as a hyphen. Dashes are used to separate or connect sentence elements whereas hyphens are used to separate or connectword elements.
Many times a hyphen is misused in writing. Here are the proper times when a hyphen should be used.
When to use a Hyphen
To Separate the Parts (When Spelling Out Numerals) of a Compound Number from Twenty-One to Ninety-Nine
thirty-six inches to a yard.
To Express Decades in Words
To Separate (When Spelling Out Numerals) the Numerator from the Denominator of a Fraction, Especially a Fraction that is used as an Adjective
One-third cup of milk
A three-fifths majority
To Combine the Elements of a Compound Modifier when used before the Noun it modifies
They engaged in hand-to-hand combat.
They fought hand to hand.
They endured a hand-to-mouth existence.
They lived hand to mouth.
When not to use a Hyphen
Do not use a hyphen to indicate a fraction if either the numerator or the denominator is already hyphenated.
Do not hyphenate compound nouns indicating chemical terms, military rank, or certain governmental positions.
Do not hyphenate a compound modifier that includes an adverb ending in -ly even when it is used before the noun.
His loose-fitting jacket
A carefully guarded secret
New truth is always a go-between, a smoother-over of transitions.
Nineteen hundred forty-three.
An expert who is well known.
A well-guarded secret.
Long- and short-term memory.