a, an

Use the article a before consonant sounds: a historic event, a one-year term (sounds as if it begins with a w). a united stand (sounds like you).

Use the article an before vowel sounds: an energy crisis, an honorable man (the h is silent). an NBA record (sounds like it begins with the letter e), an 1890s celebration.

a-

The rules of prefixes apply, but in general no hyphen.
Some examples: achromatic, atonal.