https://www.curso-ingles.com/practicar/ejercicios/past-simple
https://www.cerebriti.com/juegos-de-verbos+irregulares/tag/mas-recientes/
http://www.macmillandictionaries.com/features/verb-wheel/
http://www.eslgamesplus.com/past-simple-irregular-verbs-esl-grammar-interactive-activity-online/
http://www.bbc.co.uk/schools/starship/english/games/blast_the_rocket/big_no_sound/full2.shtml