Explicação com vídeo: https://www.youtube.com/watch?v=EZopcVLDCHg
Link 1: https://www.ego4u.com/en/cram-up/grammar/past-perfect-simple/exercises
Link 2: http://www.focus.olsztyn.pl/en-exercises-past-perfect-test-1.html#.WFL9MaIrKqA
Link 3: http://www.englisch-hilfen.de/en/exercises/tenses/past_perfect.htm