present perfect / simple past

PRESENT PERFECT:

a) I have already arrived home.

SIMPLE PAST:

b) I arrived home two hours ago.

PRESENT PERFECT:

c) I have been in Barcelona many times/several times/a couple of times/once/(no mention of time).

SIMPLE PAST:

d) I arrived in Palma last year/three years ago/in 1989/in 1985 and 1989/when I was sixteen years old.

In (a): I arrived home at an unspecified time in the past (sometime before now).

In (b): I arrived home at a specific time in the past (two hours ago).

The present perfect expresses an activity that occurred at an unspecified time (or times) in the past, as in (a) and (c).

The simple past expresses an activity that occurred at a specific time (or times) in the past, as in (b) and (d).