This website explains the past perfect again:
http://www.englishpage.com/verbpage/pastperfect.html
exercises
http://www.ego4u.de/de/cram-up/grammar/past-perfect-simple/exercises
http://www.perfect-english-grammar.com/past-perfect-exercise-1.html
http://www.englisch-hilfen.de/en/exercises/tenses/past_perfect.htm
http://www.englisch-hilfen.de/en/exercises/tenses/past_perfect_negation.htm
http://www.englisch-hilfen.de/en/exercises/tenses/past_perfect_questions.htm
http://www.perfect-english-grammar.com/past-perfect-exercise-2.html
http://www.englishgrammarsecrets.com/pastperfect/exercise1.html
http://www.englishgrammarsecrets.com/pastperfect/exercise2.html
http://www.englishgrammarsecrets.com/pastperfect/exercise4.html
past perfect or simple past?
http://www.englisch-hilfen.de/en/exercises/tenses/past_perfect_simple_past.htm
http://www.perfect-english-grammar.com/past-perfect-exercise-2.html
http://www.englishpage.com/verbpage/verbs11.htm
http://www.english-4u.de/past_perfect_ex5.htm
