Modals

Have a look at this table with the main modal auxiliary verbs and related expressions and functions.

tabla-de-modales-usos-y-ejemplos

and here you have an extensive explanation in Spanish

Teoría Modales y Modales Perfectos

and now try these quizzes:

Modal Quizz with answers

and this tests:

Test on Modals and Related Expressions

Modals and Related ExpressionsANSWERS

And here you have some links to modals on the net:

http://anasenglishactivities.blogspot.com.es/2008/03/modal-verbs.html

http://www.englishexercises.org/makeagame/viewgame.asp?id=2229

http://www.agendaweb.org/verbs/modals-exercises.html

http://www.ego4u.com/en/cram-up/grammar/modal-verbs/exercises

http://www.perfect-english-grammar.com/modal-verbs-of-obligation-exercise-1.html

More exercises with answers:

VARIED MODAL VERBS EXERCISES

VARIED MODAL VERBS EXERCISES II