On the surface, the three articles in the English language (a, an, and the) are uncomplicated little runts, and for this they should be congratulated. They’re so small that they barely even exist and therefore should require the minimum possible amount of brain cell activity in order to use and understand. But things can’t be that easy, can they? Well, they can. They just aren’t because of our incurable need to establish and follow odd, nonsensical conventions.
Adhering to the basic elementary school rule, we use an before words that begin with vowel sounds. This makes an a little bit special because it has no basis in the written word. It exists solely to aid in graceful pronunciation by providing us with a convenient little consonant so that we don’t have to go stumbling over consecutive vowel sounds as if a ghost just passed through our larynx. A impossible task looks fine on paper, but it is horrible to say and requires some advanced verbal gymnastics to prevent the vowels from getting smashed together into unpossible task instead of a impossible task. We add an n to prevent this kind of awkward, perilous voweling. And if you are currently experiencing any perilous voweling, you should get that examined by a local witch doctor immediately. An elixir of peppered pond water and an exorcism should be prescribed, to be taken three times fortnightly.
The premise behind an is simple enough, but in the grand tradition of everything being the worst, a and an get all tangled and confused for no reason when it comes to the word historic because people insist on using an even though it makes no sense.