A or An? Its a question that is now confused with the increasing amount of communication done purely in text. In spoken English, it is quite simple: 'A' before a noun, 'An' before a vowel. Spoken is the key word here - the spoken language doesn't care if its 'ewe' or 'you', 'won' or 'one'. In written English, the vowels are defined as 'a', 'e', 'i', 'o' and 'u'. Thus, two words that sound the same may mistakenly have two different ways to write them: 'an ewe' and 'a you', or 'a won' and 'an one'. Try saying these word pairs out loud and it is immediately apparent that 'an ewe' and 'an one' sound wrong. There are other examples that exist, 'http' is typically pronounced 'eitch-tee-tee-pee' and thus starts with a vowel sound - 'a http' does not sound right at all. Likewise 'SGML' is pronounced 'ess-jee-em-ell' and sounds wrong with 'a' in front of it.

The key to deciding which indefinite article is correct is to say the word and listen to the first sound. The a/an distinction was created for the spoken language. Even though the rules do not fit well with many people's ideas of how to write text, this is how it is meant to be.

Webster gives several very good examples of this in a and an.