A versus An

"A" goes before words that begin with consonants.

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

"An" goes before words that begin with vowels:

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

Exceptions

Use "an" before unsounded "h." Because the "h" hasn't any phonetic representation and has no audible sound, the sound that follows the article is a vowel; consequently, "an" is used.

• an honorable peace
• an honest error

When "u" makes the same sound as the "y" in "you," or "o" makes the same sound as "w" in "won," then a is used. The word-initial "y" sound ("unicorn") is actually a glide [j] phonetically, which has consonantal properties; consequently, it is treated as a consonant, requiring "a."

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

For more information, please visit the page on the OWL.