http://www.shertonenglish.com/resources/es/questions.php
http://learnenglish.britishcouncil.org/en/grammar-exercises/question-forms-su…
http://www.esl-lounge.com/student/grammar/3g26-subject-object-questions.php
http://www.englishexercises.org/makeagame/viewgame.asp?id=719