should, must, have to

grammar
http://www.aulafacil.com/Ingejerc/Lecciones/Lecc26.htm
exercises
http://www.aulafacil.com/Ingejerc/Lecciones/Ejerc26.htm

http://www.englishexercises.org/makeagame/viewgame.asp?id=6270

http://www.englishexercises.org/makeagame/viewgame.asp?id=3401