Rules here :
indefinite article: a and an
definite article: the
Exercises here :
http://englishpage.com/articles/index.htm
http://sdoyouspeakenglish.blogspot.com.by/p/blog-page_66.htmlhttp://sdoyouspeakenglish.blogspot.com.by/p/a-party-this-is-css-stylesheet-used-in.html
http://sdoyouspeakenglish.blogspot.com.by/p/tests-articles-3.html