An should be used before countable nouns that talk about one person or thing, and begin with a vowel sound (a, e, i, o, u).
- Do you want an apple or a banana?
(Apple, a-pel – vowel sound. Banana, bah-nan-a. No vowel sound.) - I have an egg.
(eh-g) - She is eating an ice-cream.
(eye-scream)
The sound is important not the spelling.
- She is an hour late.
(h is silent – pronounced our. No vowel sound so no a nor an.) - An umbrella.
(um-brell-a) - A university.
(you-ni-ver-sity. Vowel sound so a.)
If a word is put between the article and the noun it will affect the use of either a or an.
Compare:
- I have an egg.
(eh-g – vowel sound) - I have a fried egg.
(fr-eye-d – no vowel sound)
