hello! past perfect and simple past tenses are totally different from each other.That is;both of them happen in the past but an event happens in past perfect before simple past,for example,I had done my homework,when my mother came home.It means first I did my homework,then my mother came home.Also we can't use simple past instead of present perfect because,in simple past there is no effect of the event going on,but in present perfect the event begins in the past and still goes on.
