Modal Verbs Further Explanation

http://imagenes.mailxmail.com/cursos/imagenes/7/9/modal-verbs_28297_3_1.GIF

http://www.myteacherpages.com/webpages/alfonsolopez/files/Modal%20verbs.pdf

Must or Have to

https://elt.oup.com/student/headway/int/grammar/unit04/hwy_int_unit04_2?cc=gb&selLanguage=en&mode=hub

https://elt.oup.com/student/englishfile/intermediate3/grammar/file04/nef_int_grammar04_b03?cc=gb&selLanguage=en&mode=hub

https://elt.oup.com/student/headway/preint4/grammar/unit08/hwy_preint_unit08_1?cc=us&selLanguage=en

http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-49540.php

http://www.ejerciciodeingles.com/ejercicios-verbos-modales-must-have-to-expresar-obligacion-rellenar-huecos/

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