Modal verbs of necessity: must, have to, have got to

http://web2.uvcs.uvic.ca/courses/elc/studyzone/330/grammar/330-modals-necessity.htm

http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-67368.php

http://www.englishgrammarsecrets.com/musthaveto/menu.php

http://www.englishclub.com/grammar/verbs-modals-have-to-must-not-quiz.htm