useful websites to study articles

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

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

http://www.learn4good.com/languages/evrd_grammar/article_ex.htm

http://a4esl.org/q/h/fb-lb-aanon.html

http://grammar.ccc.commnet.edu/grammar/quizzes/cross/articles_quiz.htm