Unlike the Definite Article The, A and AN refer to someone or something whose precise identity is not specified. And, although they are among the most common words in the English language, confusion still arises as to which should be used when. So here’s a reminder.
A is used:
(i) before all consonants: a woman, a tree, a rock.
(ii) before an aspirated h: a horse, a hero, a humorist.
(iii) before the letter u when sounded like ‘you’: a unit, a use, a union.
(iv) before a diphthong eu: a European, a eulogy.
(v) before words beginning with y: a year, a yellow balloon, a youth.
AN is used:
(i) before a vowel sound: an animal, an example, an umbrella.
(ii) before a mute h: an hour, an honest woman, an historian.
See, it’s as clear as mud. Now, I wonder who is going to be first to ask me, is a diphthong the same as a bikini thong? Hmm…