VISIT SOME WEBPAGES OF MODAL VERB EXERCISES.
http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-29834.php
http://www.ejerciciodeingles.com/ejercicios-verbos-modales-expresion-obligacion-ingles-rellenar-huecos/
http://www.englishgrammarsecrets.com/musthaveto/menu.php
http://www.tolearnenglish.com/free/news/printmatchmodaux.php