Common prefixes, suffixes, and combining forms (see lists below) generally are joined to words without a hyphen. Use a hyphen when adding a prefix, suffix, or combined form creates an ambiguous word (as in the case of homonyms) or a word that would make the reader hesitate.
You can re-cover the chair.
Can you recover the money?
He re-sided the house.
He resided in Des Moines.
Common prefixes joined to words without a hyphen:
mid- (except months)
super- (unless it means very)
*except multi-ply (which could be confused with multiply)
Use a hyphen with the prefix ex-.
Use a hyphen when the main word element is capitalized or is a figure or a word combination.