Use "an" before a word that starts with a vowel SOUND, and "a" before a word that starts with a consonant SOUND — this is about pronunciation, not spelling. Most of the time that lines up with the letter itself (a dog, an apple), which is why people learn it as a spelling rule — but the real rule is about sound, and that's what explains every exception.
Because the rule is about sound, not the letter on the page, some words break the "obvious" pattern:
Say the next word out loud (or say the acronym letter-by-letter if it's an abbreviation). If the very first sound is a vowel sound (a, e, i, o, u sound) — use "an". If it's a consonant sound — use "a", even if the spelling looks like it should be the other way.
Both are used, but "a historic" is more common in modern English since the H is usually pronounced. "An historic" is an older style, still seen when the H is spoken softly or dropped in some accents.
It's about sound, not spelling. "MBA" said aloud starts with "em" (a vowel sound), so it takes "an". "University" is pronounced "yoo-niversity" — Y is a consonant sound — so it takes "a" despite starting with the letter U.
