a / an / one

Use one when the number is important; when you want to emphasize that it is only one (and not two or three or more):

  • One of these eggs is rotten, but the others are OK.
  • I wanted to buy three CDs, but I didn’t have enough money, so I bought only one.

    In all other cases, when the fact of being “one” is not important, use a / an:

  • I had an omelet for breakfast.
  • I bought a new CD yesterday.

    What about the difference between a and an? We use an before words beginning with a vowel sound, and a before all other words:

  • an apple / a banana
  • an ice cream cone / a piece of cake
  • an egg / a carrot
  • an omelet / a steak
  • an umbrella / a uniform

    (because “uniform” is pronouncedyuniform)

  • an hour / a hat

    (we use an with hour because the H in hour is silent, but the H in hat is not)