I've noticed a number of recent questions on ELL that request advice using articles. To avoid having to list the basic rules of use, I would like to propose that the current tag wiki:
Article is the term used for a modifying word that introduce a noun phrase, and that implies that:
- The thing referred to is not specific (e.g. she bought me a book)
- The thing referred to has already been mentioned (e.g. she bought me the book I wanted to buy last summer)
In the first case, the article is the indefinite article; in the second case, the article is the definite article.
is extended with @DamkerngT. 's answer here. This change would make the answer easier to find.