http://perso.wanadoo.es/autoenglish/gr.must.i.htm

http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-28258.php

http://cla.uniss.it/inglese/Grammar/Verbi/Modals/Esercizi/esercizio2.html#2 (but the instructions are in Italian!)

you can access this forum from home