Explicação com vídeo: https://www.youtube.com/watch?v=7Mni3yDpIWo
Link 1: https://www.ego4u.com/en/cram-up/grammar/simpas-pasper/exercises?ex04
Link 2: http://www.english-4u.de/past_perfect_ex5.htm
Link 3: http://speakspeak.com/english-grammar-exercises/upper-intermediate/past-simple-v-past-perfect
