Now do the following exercises.
http://www.perfect-english-grammar.com/modal-verbs-of-ability-exercise-1.html
http://www.perfect-english-grammar.com/modal-verbs-of-obligation-exercise-1.html
http://www.focus.olsztyn.pl/en-modal-verbs-practice.html#.VpuycfnhDIU
http://www.usingenglish.com/quizzes/106.html