A few rules and some examples I found on Englishplus.com/grammar:

Hyphenated Compound Words

Hyphens are used internally in some compound words to separate the words forming the compound word.


Examples: merry-go-round editor-in-chief

When unsure of the hyphenation of such words, check a dictionary. Usage may vary. As some words are more widely used, the hyphen in dropped. For example, in the early 1800's the word blackbird was usually spelled black-bird. Now the hyphen has been dropped.


Hyphens connect the words of a compound modifier that comes before the word being modified. Hyphens are not used this way with compound parts ending in -ly or made up of proper nouns or proper adjectives.


Incorrect: He is a well respected man.
Correct: He is a well-respected man.
(A compound modifier before the noun.)

Incorrect: That man is well-respected.

Correct: That man is well respected.
(The modifier follows the noun, no hyphen.)

Incorrect: That was a badly-punctuated sentence.

Correct: That was a badly punctuated sentence.
(Modifier ends in -ly, no hyphen.)

Incorrect: The South-American rain forest is home to hundreds of species of hummingbirds.

Correct: The South American rain forest is home to hundreds of species of hummingbirds.
(Modifier is proper, no hyphen.)