Verb to be

http://www.eclecticenglish.com/grammar/PresentSimpleToBe1A.html
http://www.agendaweb.org/exercises/verbs/to-be-forms
http://www.myenglishpages.com/site_php_files/grammar-exercise-be-present.php
https://www.englisch-hilfen.de/en/exercises/tenses/be3.htm