co-

Use a hyphen when forming nouns, adjectives, and verbs that indicate occupation or status. In other combinations, do not use the hyphen unless a new word would be formed.

co-author. co-pilot. coworker. co-star. co-chairman.

coed. coeducation. coequal. coexist. coexistence. cooperate. cooperative. coordinate. coordination. co-op.

Consult Webster’s for instances not mentioned above.