The present perfect is used to indicate a link between the present and the past. The time of the action is before now but not specified, and we are often more interested in the result than in the action itself.
THE PRESENT PERFECT IS USED TO DESCRIBE
An action or situation that started in the past and continues in the present. I have lived in Bristol since 1984 (= and I still do.)
An action performed during a period that has not yet finished. She has been to the cinema twice this week (= and the week isn't over yet.)
A repeated action in an unspecified period between the past and now. We have visited Portugal several times.
I have walkedI haven't walked
Have I walked?
You have walked
You haven't walked
Have you walked?
He, she, it has walked
He, she, hasn't walked
Has he, she, it walked?