http://englishstandarts.blogspot.com/2012/01/perfect-tenses.html
http://www.focus.olsztyn.pl/en-present-perfect-tense-exercises-with-answers.html#.W7-aLNczbIU
http://www.focus.olsztyn.pl/en-exercises-past-perfect-test-1.html#.W7-g-dczbIU
http://www.focus.olsztyn.pl/en-future-perfect-exercises.html#.W7-o4tczbIV