For example: a pet, a door, a green onion, a cat, a hysterical joke.
Use “A” when u makes the same sound as the y in you: a union, a unicorn, a used napkin, a usability study. Use “A” when o makes the same sound as w in won: a one-legged man. Use “A” when eu or “e” makes the same sound as y: a European trip, a ewe lamb Do not use “A” when the h is silent.
For example: an apple, an elbow, an Indian.
Use “An” before a silent h: an hour, an honorable peace, an honest error. Use “An” before words that are spelled with consonants but pronounced with vowel sounds: an MBA.
Some American writers will also use “an” with longish words (three or more syllables) beginning with H, where the first syllable isn’t accented: “a hypothesis,” “an habitual offender. "
