A versus An

"A" goes before all words that begin with consonants.

• a cat
• a dog
• a purple onion
• a buffalo
• a big apple

With one exception: Use "an" before unsounded h.

• an honorable peace
• an honest error

"An" goes before all words that begin with vowels:

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

With two exceptions: When the same sound as the y in you, or o makes the same sound as w in won, then a is used.

• a union
• a united front
• a unicorn
• a used napkin
• a U.S. ship
• a one-legged man