Hyphens—not to be confused with dashes—are a part of some compound words. This is the universally accepted use of the hyphen.
Well-versed
Cross-eyed
Mother-in-law
Full-time
Note: Whether a compound word has a space, a hyphen, or neither depends on common usage and how long the compound word has been in use. To decide whether or not to use a hyphen, consult your preferred dictionary, even for the examples above.
The MLA Style Manual and The New York Times Manual of Style and Usage specify that a hyphen does not link an adverb ending in -ly with a modifier.
The AP Stylebook, the Publication Manual of the APA, The Chicago Manual of Style, the MLA Style Manual, and The New York Times Manual of Style and Usage specify omitting the hyphen in a modifier when it comes after the noun.
Note: The AP Stylebook excepts this rule when the modifier follows a form of “to be.”
The Chicago Manual of Style and The New York Times Manual of Style and Usage specify that if the meaning is clear, a hyphen does not link a compound modifier.
See Also: En Dashes