A - AN

If you have to use a or an, depends on the sound the word begins with.

If the first sound is a consonant you have to use a.

a sandwich


If the first sound is a vowel you have to use an.

an apple
  ice - cream


an hour - because the first letter is silent and the word starts with an au-sound.
a uniform, a university - because the first letter starts with a ju-sound.


Exercise 1 Exercise 2 Exercise 3