Topix Underground

"The choice between "a" and "an" depends on the sound that follows, not just the letter. "An" is typically used before words that start with a vowel sound (a, e, i, o, u), while "a" is used before words that start with a consonant sound. However, some words beginning with "u" are pronounced with a "y" sound at the start (like "you"), which is a consonant sound. In these cases, "a" is correct instead of "an."
For example:
"A university" – The "u" here sounds like "yoo," starting with a "y" consonant sound, so "a" is used.

"A unicorn" – Same thing, "yoo-nicorn," begins with a consonant sound.

Compare to "an umbrella" – The "u" sounds like "uh," a vowel sound, so "an" fits.

The rule hinges on pronunciation, not spelling. That’s why "an" isn’t automatically used before all "u" words—some just don’t sound like they start with a vowel."