In the English language, an indefinite article is a word used as a preface to a countable or uncountable noun.
As with several other spoken languages, the indefinite article construct is not exhaustive:
a and an are used with countable nouns.
Example: "That body of water is a lake, not an ocean."
some is used with uncountable nouns.
Example: "I'm going out to get some air."
A is used to preface words that begin with consonant sounds, while an is used to preface words that begin with vowel sounds. Some can be used with either.
"We demand rigidly defined areas of doubt and uncertainty!"
Though this is one of the simplest English grammar concepts to learn, it is also one that the most people have trouble learning, especially people learning English after lacking an indefinite article in their native language, as is the case with native speakers of Japanese. One of the most common gaffes by users of the indefinite article is using "an" to preface words that begin with the letter H, such as "an historic," or "an hysteric." Usually, this is caused by the user not making a distinction between written and spoken words, as some people pronounce words like "historic" as "-istoric," and when writing those words and words like them, develop a reflex to reach for the "an" as if the writer were speaking.
In middle English, "an" was the only indefinite article. Also, according to Fowler's, it is acceptable to use "a" to preface certain words that begin with vowels, such as "a one" or "a unit." However, words like "one" and "unit" don't start with vowels when spelled phonetically, which explains that exception. "An" has some exceptions, as well, such as "an hour," "an honest person," and so forth. In fact, when it comes to the spoken word, just about every regional dialect in the English-speaking world has its own rules for indefinite articles. This writeup is primarly concerned with the written word, which, as far as English goes, seems to be universal, apart from the o/ou, e/ae, and z/s differences between American and British English. It's been mentioned that "an historic" is correct when used in written British English, but if that's true, then wouldn't combinations like "an homosexual," "an herpetologist," and "an home" also be correct? It seems implausable, but the answer is apparently yes.
According to our resident grammarianGritchka, "an historic" is correct because the first syllable of "historic" is entirely unstressed (in certain dialects, most popularly in British and Australian English), which would permit "an" as a valid preface. "An honest person" works in much the same way. Given the relative weirdness that English possesses when compared to more structured languages (such as French or German), there seem to be more than a few exceptions to every rule. This issue in particular seems to go through phases, as apparently "an historic" is only just starting to become archaic in Britain, where it is still considered correct, whether written or spoken.
"Oh. Well, that makes sense."
There doesn't seem to be any grey areas concerning the usage of "some."
Growing up, as I did, in Michigan, and then moving to Louisiana in my early twenties, I was consistently jarred by people saying "an -istoric" rather than "a historic," and I'd always try not to look for it but I'd end up hearing it, or seeing it on the signs of antique shops, nevertheless. It seemed so alien. It drove me crazy for the first year I lived there, after which time I made a calculated effort to ignore it.
This is my grammar pet peeve. There are many like it, but this one is mine. No, it's not my life, I've gotten over being annoyed so easily. But it still doesn't read correctly. When I see it written, it's as though the writer posted a "watch your step" sign in a poorly lighted room; you see it, and then grind to a halt and look thither and yon for the errant step, or in this case, words that don't belong together.
Sources:
http://www.scribe.com.au/tip-w005.html
http://www.grammartips.homestead.com/historical.html