"I saw a great play last week." (past simple—it is completed and completely in the past) "I have seen the 'Mona Lisa.'" (present perfect—it happened during my visit to Paris, and it is possible I'll see it again before I die) "I had seen the movie before I read the book for my university class test on 'Hamlet.'" (past perfect—two things happened in the past, and the "had seen" thing happened before the second thing)

case #1 - Hey, look at my new iPod giga! - I have seen it already. case #2 - Hey, look at my new iPod giga! - I had seen it last week when we had a dinner at your house. I could be mistaken ;)