There are only three articles in English: a, an, the.
There are two types of articles: indefinite “a” and “an” or definite “the”.
John: I had a sandwich and an apple for lunch.
John says “a sandwich” “an apple” because this is the first time he talks about them.
A and An are the indefinite articles. They refer to something not specically known to the person you are comunicatig with.
A and An are used before nouns that introduce something or someone you have not mentioned before.
EX: I saw an elephant this morning.
I ate a banana for lunch.
A and An are also used when talking about your professions.
I am an English teacher
I am a lawyer.
You use “a” when the noun you are referring to begins with consonant sound example. Ex: A pencil
http://www.grammarbank.com/indefinite-articles.html http://www.englisch-hilfen.de/en/exercises/nouns_articles/article_a2.htm
