http://www.roadtogrammar.com/presperfect/
http://www.aulafacil.com/Ingejerc/Lecciones/Lecc20.htm
http://www.saberingles.com.ar/curso/lesson21/06.html
http://web2.uvcs.uvic.ca/elc/studyzone/410/grammar/ppvpast1.htm
http://www.ejerciciodeingles.com/ejercicios-uso-present-perfect-past-simple-rellenar-huecos/
http://www.ejerciciodeingles.com/ejercicios-present-perfect-past-simple-quiz/
http://www.ejerciciodeingles.com/ejercicios-utilizar-present-perfect-past-simple-contexto/
http://www.englishpage.com/verbpage/verbs5.htm
http://www.usingenglish.com/quizzes/228.html
http://www.clafoti.com/imagenes12/pres_past.htm
http://english-zone.com/verbs/prsperf4.html
http://www.oup.com/elt/global/products/englishfile/preint/a_grammar/file04/grammar04_01a/
http://www.oup.com/elt/global/products/englishfile/preint/a_grammar/file04/grammar04_a02/
http://www.oup.com/elt/global/products/englishfile/preint/a_grammar/file04/grammar04_b01/
http://www.oup.com/elt/global/products/englishfile/preint/a_grammar/file04/grammar04_b02/