There are only 3 articles in English: a, an and the, but they probably cause more grammar problems than any other words. Here are a few good lessons on articles use that include explanation, examples, and practice from the University of Victoria ESL department.
http://web2.uvcs.uvic.ca/elc/studyzone/410/grammar/410-articles-a-an-the.htm
http://web2.uvcs.uvic.ca/elc/studyzone/490/grammar/articles-general.htm
http://web2.uvcs.uvic.ca/elc/studyzone/490/grammar/articles-fine-tuning.htm
Additional on-line practice:
http://a4esl.org/q/f/x/xz38mvt.htm
http://a4esl.org/q/h/vm/fampeople.html
http://a4esl.org/q/h/9901/cw-articles.html
Worksheets and answers
http://www.eslpdf.com/esl_articles.html
http://www.eslflow.com/prepositionlessonplans.html (scroll down to "articles)
Enjoy!
