THE DIFFERENCE BETWEEN THE PRESENT PERFECT AND THE 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.

Remember:

We use the past simple for past events or actions which have no connection to the present.

We use the present perfect for actions which started in the past and are still happening now OR for finished actions which have a connection to the present.

We CAN'T use the present perfect with a finished time word:

NOT: I've been to the museum yesterday.