Dictionary.net

(sometimes called indefinite article. Before all consonants except silent h, use a; a history, a historian, though some still write an before h in unaccented syllable, but an hour: before all vowels except eu, u, use an; an ulcer, but a unit, a eulogy; also a one. Placed after many, such, what, or any adj. preceded by how, so, as, too. Used with apparent plurals of number, a dozen men =a dozen of men; also with pl. adjj. few, good many, great many). (Unemphatic substitute for) one, some, any; one like (a Daniel); (after all of, many of, &c.) the same (all of a size); (distrib.) each (40 a year, where a is orig. =foll.). [old English]