modal verbs

may/may not

http://www.english-grammar-lessons.com/maymight/exercise1.swf

 

must/have to

http://www.englishpage.com/modals/interactivemodal2.htm

 

could/might/should/would

http://www.englishpage.com/modals/interactivemodal6.htm

 

might/might not

http://www.english-grammar-lessons.com/maymight/exercise2.swf

 

practise with will/won't

http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-15268.php

http://www.isabelperez.com/happy/tenses/exercises/will_1.htm