an

The forms of the indefinite article are good examples of what can happen to a word when it becomes habitually pronounced without stress. An is in fact a weakened form of one; both an and one come from Old English ān "one.” In early Middle English, besides representing the cardinal numeral "one,” ān developed the special function of indefinite article, and in this role the word was ordinarily pronounced with very little or no stress. Sound changes that affected unstressed syllables elsewhere in the language affected it also. First, the vowel was shortened and eventually reduced to a schwa (ə). Second, the n was lost before consonants. This loss of n affected some other words as well; it explains why English has both my and mine, thy and thine. Originally these were doublets just like a and an, with mine and thine occurring only before vowels, as in Ben Jonson's famous line "Drink to me only with thine eyes.” By the time of Modern English, though, my and thy had replaced mine and thine when used before nouns (that is, when not used predicatively, as in This book is mine), just as some varieties of Modern English use a even before vowels (a apple).