2NA Conditionals

http://random-idea-english.blogspot.com.es/2013/01/inversion-in-conditionals.html

https://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-89063.php

http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv342.shtml

http://www.esl-lounge.com/student/grammar/4g14-advanced-mixed-conditionals-gap-fill-exercise.php