https://www.ego4u.de/de/cram-up/grammar/simpas-preper/exercises
http://www.englisch-hilfen.de/uebungen/zeiten/simple_past_present_perfect4.htm
https://www.ego4u.de/de/cram-up/grammar/simpas-preper/exercises?ex04
https://www.ego4u.de/de/cram-up/grammar/simpas-preper/exercises?ex05
http://www.englishpage.com/verbpage/verbs5.htm
https://www.ego4u.de/de/cram-up/grammar/simpas-preper/exercises?ex06
http://www.perfect-english-grammar.com/past-simple-present-perfect-4.html
https://www.ego4u.de/de/cram-up/grammar/simpas-preper/exercises?ex07
https://www.ego4u.de/de/cram-up/grammar/simpas-preper/exercises?ex08
http://www.kico4u.de/english/uebungen/aktiv/preperfmix.htm
http://www.usingenglish.com/quizzes/228.html
