Used before words that begin with a vowel sound to refer to a single, non-specific person or thing. It signals that the listener does not yet know exactly which one you mean.
From Old English *ān*, meaning “one,” which served as both the number and the indefinite article. Over time, *an* remained before vowel sounds, while *a* developed before consonant sounds.
“An” is just a worn-down version of “one”—you can still hear it in phrases like “an apple” becoming “one apple.” English keeps two spellings, *a* and *an*, not for meaning but for smooth pronunciation. It’s grammar that exists purely to keep your mouth happy.
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