PAST PERFECT
http://www.englishpage.com/verbpage/pastperfect.html
http://www.perfect-english-grammar.com/past-perfect.html
http://www.perfect-english-grammar.com/past-perfect-use.html
http://learnenglish.britishcouncil.org/en/english-grammar/verbs/past-tense/past-perfect
http://www.englishclub.com/grammar/verb-tenses_past-perfect_s.htm
http://www.englishclub.com/grammar/verb-tenses_past-perfect_u.htm
http://www.ego4u.com/en/cram-up/grammar/past-perfect-simple
http://www.perfect-english-grammar.com/past-perfect.html
http://www.perfect-english-grammar.com/past-perfect-use.html
http://learnenglish.britishcouncil.org/en/english-grammar/verbs/past-tense/past-perfect
http://www.englishclub.com/grammar/verb-tenses_past-perfect_s.htm
http://www.englishclub.com/grammar/verb-tenses_past-perfect_u.htm
http://www.ego4u.com/en/cram-up/grammar/past-perfect-simple