MODAL VERBS

http://www.englishpage.com/modals/modalintro.html

http://english-language.webpark.pl/modal_verbs.html

http://wwwedu.ge.ch/cptic/prospective/projets/anglais/exercises/intromodal.html

http://depts.gallaudet.edu/englishworks/grammar/modals.html

http://esl.fis.edu/grammar/rules/modal.htm