[Dr. Jamshid Ibrahim]

We are talking here about present perfect simple only. Present perfect simple can be used for an action which is complete just like past simple. In present perfect simple the present is relevant (that's why its name is present perfect although it might be misleading). In the following cases present perfect simple can refer to an action which is complete. Other functions of present perfect simple are left out here:

1. The speaker is interested in the present ie results not in a specific time in the past:
I have broken my leg. Plays a role in the present.

2. provides an explanantion for a present situation:
Why are you tired?
I am tired because I have worked hard.

3. Actions which come to an end at the moment of speaking:
I haven't eaten Korean food for a long time.

4. Changes which are complete:
They have opened a new restaurant in the city.

5. Present perfect can start a cnversation because the time reference is not defined. The switch to past simple is followed soon:
Where have been?
I have been to the cinema.
What was the film?

5. American English can use past simple instead of present perfect:
I didn'tt see her yet. AmE
I haven't seen her yet. BE

This would answer your bother's question why present perfect simple is used for actions which are complete like past simple.