A / AN

The Indefinite Article – A / AN

We use A/AN with:

1. Singular nouns and the first time we refer to a person, animal or thing.

• a child
• an elephant
• a television

2. We don't use A/AN with possessive pronouns, demonstratives or cardinal numbers.

• My shirt is dirty.
• This car is expensive.
• One person is in the reception.

3. We use ONE (or more) instead of A/AN when the number is important.

• There is only one exit from the airport.

What is the difference between A and AN?

A is used when the next word begins with a consonant sound (b, c, d, f, g, h, j, k etc).

• a book
• a table
• a clock
• a university (because the beginning of university sounds like YOU-niversity)

AN is used when the next word begins with a vowel sound (a, e, i, o, u).

• an apple
• an elephant
• an umbrella
• an hour (because the H is silent)

http://www.grammar.cl/english-games/a-vs-an-difference.htm                

http://www.grammarbank.com/indefinite-articles.html

http://www.englisch-hilfen.de/en/exercises/nouns_articles/article_a2.htm