Susana is right, up to a point, that we use 'a' before words that begin with consonants ( b, c, d, f, g and so on) and 'an' before words that begin with a vowel (a, e, i, o, and u) . But it isn't quite that simple. The important thing to remember is that this is a rule about the SOUND of words, not the spelling. So there are some exceptions to the simple vowel/consonant rule: 1. We use 'an' before words which begin with silent consonants : for example, 'an hour' and 'an honour'. 2. We use 'an' before certain letters and numbers which are pronounced with initial vowel sounds: for example 'an MBA' 'an FBI agent' 'an 18-year-old girl'. 3. We use 'a' before words beginning with vowels that are pronounced like the consonant 'y': for example 'a euro' 'a university'

"an" is used before words that begin with vowels (a , o , u , i , e). for example : an apple. "a" is used before the rest of alphabet (Consonants). for example : a ball.

You use a when the next word starts with consonant (b, c, d, f, g, and so on) and use an when the word starts with a vowel (a, e, i, o, u). Eg. A computer, A box, A pen AN eye, AN idea, AN elephant