Vídeo com explicação: https://www.youtube.com/watch?v=23sIBk6gL4w
Link 1: http://www.englishgrammar.org/continuous-tense-worksheet/
Link 2: http://www.myenglishpages.com/site_php_files/grammar-exercise-past-progressive.php
Link 3: http://www.really-learn-english.com/past-progressive-exercises.html
