We use Present Perfect:
to look at the past from a present perspective:
She has worked here for several years.
They have not seen her since 1994.
to describe a past event that has a consequence or result in the present:
I have done my homework.
She has forgotten her keys.
when the exact date or time of a past event is not important or specified:
I have been to France.
She has met Frances before.
Past Simple is used:
when the exact date or time is specified:
I met her last summer.
I did it two days ago.
when we are not thinking of the present consequences of the past event:
I lost my keys.
I bought some milk.
We use Present Simple to describe a present situation now: