(i) Use 'an' before a word beginning with a vowel sound (Vowel sounds are: a, e, I, o, u) :
An obvious answer
(ii) Use 'an' before an 'h' that is not pronounced, i.e, an 'h' which is silent/unpronounced.
An honest answer
(iii) Use 'an' before an abbreviation that is pronounced with a vowel sound at the beginning.
An AU organisation
An MP3 file
An SOS call
(iv) Use 'a' before a 'u' that is pronounced like 'you', i.e, that spelt 'u' at the beginning but pronounced like 'you' /ju:/.
A unique person
A universal character
A University course
(v) Use 'a' before any word beginning with a consonant sound, i.e, those words that do not begin with the vowel sounds.