Verbo to be

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

http://www.juntadeandalucia.es/averroes/ceip_san_tesifon/recursos/ingles/act_interactivas/cuestionarios/tobe0.htm

http://www.oup.com/elt/global/products/englishfile/elementary/a_grammar/file01/grammar01_a01/