Present Perfect and Simple Past Tenses

To understand Present Perfect and Simple Past Tenses better ,please have a look at the links i have given below:

http://www.englishpage.com/verbpage/presentperfect.html
http://www.perfect-english-grammar.com/present-perfect-or-past-simple.html
http://web2.uvcs.uvic.ca/elc/studyzone/410/grammar/ppvpast.htm
http://www.englisch-hilfen.de/en/exercises/tenses/simple_past_present_perfect.htm