Explicação com vídeo: https://www.youtube.com/watch?v=T-CTP0JpnWc
Link 1: http://www.englisch-hilfen.de/en/exercises_list/mengen.htm
Link 2: http://www.englishgrammar.org/grammar-exercise-25/
Link 3: http://www.englishexercises.org/makeagame/viewgame.asp?id=2610
Link 4: http://www.carmenlu.com/first/grammar/countable1_1.htm