Indefinite Articles: A and An
The indefinite articles are used when we are referring to an unspecified thing or quantity. We use them when we don’t know (or don’t care) which thing we’re talking about.
There once was a sheep.
I don’t know which sheep it was – don’t know its name or where it’s from, or anything about it – so I can’t say the sheep.
We use a before nouns or adjectives which begin with a consonant, or when U sounds like a Y, or O sounds like a W.
A young puppy
A one-eyed pirate
We use an before words which begin with a vowel sound or a soft H sound.
An antique necklace
This rule also applies to acronyms.
A Society of Writers member was quoted in the article.
A S.O.W. member was quoted in the article.
An S.O.W. member was quoted in the article.
Because S sounds like it begins with a vowel (ess), an should be used in front of it.