The rule is that “a” goes before a word starting with a consonant. And “an" goes before a word starting with a vowel.

BUT the rules follows the phonetic pronunciation NOT the spelling.

some examples with pronunciation in [brackets]

"a" + consonant (both in spelling and pronunciation)

• a cat [k]
• a dog [d]
• a purple onion [p]
• a buffalo [b]
• a big apple [b]

"a" + vowel that is pronounced like a consonant

• a European [ˌjʊɚ...] say: a juropean
• a one-legged man [ˈwan] say: a won-legged man
• a union [ˈjuːn.jən] say: a junion)

"an" + vowel (both in spelling and pronunciation)

• an apricot
• an egg
• an orbit
• an uprising

"an" before consonant that is pronounced like w vowel

• an honorable person [ˈɒnəɹəbl̩] say: an onorable person)
• an honest error [ˈɒnɪst] say: an onest error