How do you know when to use the indefinite articles?

The choice of article is based upon the phonetic (sound) quality of the first letter in a word, not on the orthographic (written) representation of the letter. If the first letter makes a vowel-type sound, you use "an"; if the first letter would make a consonant-type sound, you use "a." However, even if you follow these basic rules when deciding to use "a" or "an," remember that there are some exceptions to these rules.

"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 a silent or unsounded "h." Because the "h" does not have any phonetic representation or 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 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 OWL's page on using articles.