The contrast between “simple past” and “present perfect” can be confusing for students, because the terminology is misleading.
The principal use of the simple past is to place the action expressed in the past, without reference to the present:
She worked there for six years.
She knew Bob back in the ’80s.
She earned her law degree.
She saw the elephant.
The presumption is that the action is complete, over-and-done-with—that is, “perfect”, in the grammatical sense; so in this use the simple past is perfective.
Note however that this use implies nothing about the present:
She worked there for six years ⇨ then left and came here; but last month she went back there.
She knew Bob back in the ’80s ⇨ but lost track of him for quite a while, until they met again at a conference in 2006.
The principal use of the present perfect is to place the action expressed in the past, but with some reference to the present. If the main verb expresses an activity or a state, the presumption is that the activity or state continues into the present:
She has worked there for six years ⇨ and is still working there.
She has known Bob since the ’80s ⇨ and still knows him today.
If the main verb expresses a telic action, however — an action with a goal, or one which effects a change of state — the presumption is that the action was completed before the present; in this use the present perfect, too, is perfective:
She has earned her law degree ⇨ and is therefore competent to act on your behalf.
She has seen the elephant ⇨ and the experience still exerts a profound effect.