class 8: passive with modals

http://www.learnamericanenglishonline.com/Green%20Level/Green_Level_Quiz_9_passive_modals.html
http://www.ego4u.com/en/cram-up/grammar/passive/exercises?05
http://www.englishexercises.org/makeagame/viewgame.asp?id=5919
http://www.englishpage.com/modals/modalforms.html
http://www.esl-classroom.com/grammar/passive6.html