Verb to be

 

http://www.aprenderinglesfacil.es/2008/03/verbo-to-be-ser-o-estar.html

http://www.eclecticenglish.com/grammar/PresentSimpleToBe1A.html

http://www.englisch-hilfen.de/en/exercises/tenses/be3.htm

http://www.agendaweb.org/verbs/to-be-exercises.html