Simple past vs. present perfect

Simple past
(a) I finished my work two hours ago
Present perfect
(b) I have already finished my work
In (a): I finished my work at a specific time in the past ( two hours ago)
In (b): I finished my work at an unspecified time in the past (sometime before now)
Simple Past
(c) I was in Europe last year/three years ago/in 1999/in 1995 when I was ten years old
Present Perfect
(d) I have been in Europe many times/several times/a couple of time/once(no mention of time)
In (a): I finished my work at a specific time in the past ( two hours ago)
In (b): I finished my work at an unspecified time in the past (sometime before now)
The Simple Past expresses an activity that occured at a specific time (or times) in the past, as in (a) and (c).
The Present Perfect expresses an activity that occured at an unspecified time (or time) in the past, as in (b) and (d).
Simple Past
(e) Ann was in Miami for two weeks
Simple Perfect
(f) Bob has been in Miami for two weeks/since May first
In (e): In sentence where for is used in a time expression, the simple past expresses an activity that began and ended in the past.
In (f): In sentences with for or since, the present perfect expresses an activity that began in the past and continues to the present.