modals

http://www.learnenglishfeelgood.com/esl-mixed-modals1.html
http://www.ilv.ucl.ac.be/gramlink-an/exercices/auxmod/auxmod01.htm
http://www.ilv.ucl.ac.be/gramlink-an/exercices/auxmod/auxmod02.htm
http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-9225.php
http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-2628.php
http://esl.fis.edu/grammar/multi/modal1.htm
http://www.english-4u.de/modal_verbs_ex3.htm
http://www.eoilangreo.net/cristina/intermediate/modals.html