http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-17482.php
http://www.montsemorales.com/gramatica/Usedto.htm
http://www.englishexercises.org/makeagame/viewgame.asp?id=1230
http://www.perfect-english-grammar.com/used-to-exercise-1.html