The use of indefinite articles in any language is dictated by the spoken tongue, not by the written one. Just as it bothers fairnymph to see that used insted of who or [/i]whom[/i] it is the bigesr pet peeve of mine to see people constantly use an in their posts where a should have gone. There seems to have evolved this secret rule along the way which apparently dictates that the indefinite article is controlled not by the word that immediately follows it, but specifically the noun it belongs to. Hence, I constantly see such creative assembly as an ripe orange or a orange bag (where we should have seen a ripe orange and an orange bag). The saddest part about this new evolution is that it is now being propagated as law by many teaching establishments. For heavens sake, go back to the roots, my learned friends.
To sum up, learn to speak first. Your speech will take care of your writing rules, where indefinite articles are concerned (yes, ithis is one of the few cases where the spoken tongue presides over the written one). We say
- an hour instead of a hour (the h stays silent in all dialects)
- an umbrella, but a used umbrella (the y sound in used is deemed a consonant - remember, speech presides)
- a historical fact and an historical fact can both be correct, as the silence of the h is fluid
a precedes consonant sounds, an precedes vowel sounds. Not the first written letter of the following word, but the first sound of that word!
That is all you have to remember