Explication:
http://www.hku.hk/french/starters/structures/06_pronoms2.htm#en
Exercices:
http://www.romanistik.uni-mainz.de/franz/test/13_conj.htm
http://www.univ-tlse2.fr/gril/Yen.html
http://elearn.mtsac.edu/bvigano/french2/exbonvoyage.htm
http://www.didieraccord.com/exercices/index.php?ex=2.2.3.2