an

The word “an” means the form of "a" used before a word that starts with a vowel sound. In plain terms, just "a" adjusted for pronunciation before vowel sounds.

“An” is pronounced AN ([ˈeən] in IPA).

Here is “an” used in a sentence: “She adopted an old dog from the shelter.”

“An” is an article.

The word “an” comes from Old English. From Old English an, meaning "one" — the same root that gave us the number "one.".

“An” is a very common word in modern English, usually learned at around the A1 level.

Use "an" before any word that begins with a vowel sound, regardless of spelling: "an apple," "an hour" (silent h), "an MP" (spoken as "em-pee"). Use "a" before consonant sounds, including vowel letters pronounced with a "y" or "w" sound: "a university," "a one-off." The rule follows sound, not spelling.

"A historic" is standard in modern English because the "h" in "historic" is pronounced. "An historic" is an older convention still used by some speakers and publications, but most current style guides recommend "a historic," "a hotel," and "a horrific."

Go by how the abbreviation is spoken aloud. Say "an MBA" and "an FBI agent," because the first letters are pronounced with vowel sounds ("em," "ef"), but "a URL" ("you-ar-el") and "a NATO summit," because those begin with consonant sounds when spoken.

It depends on pronunciation. In American English the "h" in "herb" is silent, so "an herb" is correct; in British English the "h" is pronounced, making "a herb" the standard choice. The article simply follows whichever pronunciation you use.