http://www.esl-classroom.com/grammar/shouldandmust1.html

http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-55666.php

http://www.eclecticenglish.com/grammar/Should1B.html

http://www.tolearnenglish.com/english_lessons/should-ought-to-must-have-to-exercise