Exercise Verb

http://www.englisch-hilfen.de/en/exercises_list/verbs.htm

Exercise Tense

http://www.englishpage.com/verbpage/verbtenseintro.html

Exercise Active and Passive Voice

http://www.englisch-hilfen.de/en/exercises_list/passiv.htm

Exercise Subject-Verb Agreement

http://owl.english.purdue.edu/exercises/5/13/34