The rule for 'a' vs 'an' depends strictly on the initial sound of the following word, not its written spelling:
- Use 'An' before Vowel Sounds (a, e, i, o, u sounds): an honest man (silent 'h', begins with 'o' sound), an MLA (begins with 'em' sound), an umbrella, an hour, an heir.
- Use 'A' before Consonant Sounds: a university (begins with consonant 'yu' sound), a European (begins with 'yu' sound), a one-eyed man (begins with 'wa' sound), a uniform.