A vs. AN.

Here are a few basic things to remember when asking yourself if you should use the indefinite article "a" or "an" when preceding a noun.

1- Use the indefinite article "a" before a singular noun that begins with a consonant sound.

A car
A table
A job
A bag

2- Use the indefinite article "an" before a singular noun that begins with a vowel sound.

An apple
An eye
An orange
An idea

3- Nouns that start with "H" and "U" can cause some confusion.

- If the "H" is silent, use the indefinite article "an".

An hour

- If the "H" is pronounced, use the indefinite article "a".

A helicopter

- If the "U" is pronounced as a vowel, use the indefinite article "an".

An umbrella

- If the "U" is pronounced as a consonant, such as in the sound "you", use the indefinite article "a".

A university


Here is a link to an exercise that might be of some use to you.

http://www.anglaisfacile.com/exercices/exercice-anglais-2/exercice-anglais-4547.php