PRESENT PERFECT VS PAST SIMPLE

Present Perfect Simple	Past Simple
Unfinished actions that started in the past and continue to the present: I've known Julie for ten years (and I still know her).	Finished actions: I knew Julie for ten years (but then she moved away and we lost touch).
A finished action in someone's life (when the person is still alive: life experience): My brother has been to Mexico three times.	A finished action in someone's life (when the person is dead): My great-grandmother went to Mexico three times.
A finished action with a result in the present: I've lost my keys! (The result is that I can't get into my house now).	A finished action with no result in the present: I lost my keys yesterday. It was terrible! (Now there is no result. I got new keys yesterday).
With an unfinished time word (this week, this month, today): I've seen John this week.	With a finished time word (last week, last month, yesterday): I saw John last week.