MODAL VERBS

Here's a site where you can practise modal verbs:

http://www.usingenglish.com/quizzes/398.html

http://www.usingenglish.com/quizzes/528.html

In the following page, you'll find the different degress of uncertainty expressed by modals (explanation and practise)

http://www.grammar-quizzes.com/modal1.html