http://youtube.com/watch?v=LPuRkmAIdtc
Present Perfect Simple
http://web2.uvcs.uvic.ca/elc/studyzone/410/grammar/pperf.htm
Exercises:
http://web2.uvcs.uvic.ca/elc/studyzone/410/grammar/pperf1.htm
http://web2.uvcs.uvic.ca/elc/studyzone/410/grammar/pperf2.htm
http://web2.uvcs.uvic.ca/elc/studyzone/410/grammar/pperf3.htm