Modal verbs exercises for english 4
http://www.englishpage.com/modals/interactivemodal1.htm
http://www.nonstopenglish.com/lang/es/exercise.asp?exid=917
http://www.autoenglish.org/modalverbs.htm
http://a4esl.org/q/h/0001/jb-ability.html
http://a4esl.org/q/h/0101/ig-modals.html