Think of an action as having a starting point and an end point in terms of time reference. Simple past indicates that the action was completed. Both end points are in the past. Present Perfect just says that the starting point is in the past while the end point is not clear, or of no importance, just that the action took place in the past.
I was in Paris last year -- very specific, and emphasizes that the event has ended.
I have been to Paris -- was there some time in the past (possibly many times, possibly again in the future), and thus know what you are talking about when you are talking about Paris.
I thought the earth was square (no longer think)
I have thought the earth was square (possibly still think). Whether the action has ended is not important to convey.