(a) Hyphenate a prefix joined to a proper noun or adjective:
transatlantic and transpacific.
(b) Hyphenate expressions beginning with the prefixes ex (when it means "former"), self and all, when used to form adjectives or nouns, and those beginning with quasi when used to form adjectives:
However, when self is the base word to which a suffix is added, do not hyphenate:
(c) Most words beginning with the following prefixes are written as one word: after, ante, anti, bi, co, counter, de, down, extra, infra, inter, intra, iso, macro, micro, multi, over, photo, poly, post, pre, pro, pseudo, re, retro, semi, stereo, sub, super, trans, tri, ultra, un, under and up:
However, there are many exceptions. Check the Gage Canadian Dictionary when in doubt, and see below and 2.07 Prefixes(a) for specific types of exception.
Use a hyphen when the word following the prefix begins with the same vowel as the one with which the prefix ends and when the compound’s appearance would be confusing without the hyphen:
In certain cases, use the hyphen to preserve a difference in meaning between the hyphenated and the solid compound:
(d) Write SI/metric unit compounds as one word:
(e) Hyphenate chemical terms preceded by an italicized prefix:
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