Indefinite and definite articles:

https://owl.english.purdue.edu/exercises/2/1/7

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

http://www.english-grammar.at/online_exercises/articles/article2.htm

http://www.english-grammar.at/online_exercises/articles/article1.htm

http://www.english-grammar.at/online_exercises/articles/art004.htm

http://www.english-grammar.at/online_exercises/articles/art005.htm

https://owl.english.purdue.edu/exercises/2/1/12

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