How is usage of 'a' different from 'an' in english language?

Solution:
The usage of 'a' is different from 'an' in english language in this way --

'A'  is used before -
a) a consonant : a boy, a word, a B.A. etc.
b) such vowels that have the sound of ''yu'' :  a ewe, a useful thing, a European etc
c) ''O'', whenit is sounded as in 'one'. : a one-rupee note, a one eyed person. etc

'An' is used before-
a) a vowel: an eye, an idiot, an umbrella etc
b) a consonant beginning with a vowel sound : an M.A., an honest , an S.D.O etc