O co chodzi z rezultatem w present perfect ?

Kiedy widzę jak w filmie postacie używają Past Simple w momentach kiedy coś się wydarzyło w stylu:

-"I forgot to bring my notebook" - nawet kiedy postać w "rezultacie" nie ma tego notatnika/zeszytu. To wciąż używamy Past Simple. Rezultat niby jest zapomniał i go nie ma.

Słyszałem wiele wyjaśnień, z angielskich forów, że Past Simple używamy w sytuacji gdy chcemy coś poinformować nawet kiedy jest jakiś rezultat, albo do podkreślenia przeszłości, a rezultat tego nas nie obchodzi nawet kiedy taki rezultat jest.

I właśnie ten rezultat mnie trochę miesza w głowie, i za każdym razem kiedy widzę jakieś zdanie w Past Simple albo w Prezent Perfect to od razu leci analiza ... Dlaczego to a nie na przykład to ?

I have yet to see a movie where dialogs follow all grammar rules ;-))

Yea but I wonder which one should I use. I just don't want to make a mistake.
I learn language by watching the movie or reading a book (that has those kind of situation like in movie)

For example
- Who beat you Paul ?
It was Nick and his boys - we used here past simple (even when the character see the result that his friend Paul is beaten up) because we want to emphasize the past action even when we see the result ? or am I wrong ?

you are describing a particular event from the past.
When you describe the details of an event, you use Past Simple

have you been beaten up?
yes
who beat you?
It was Nick...

Yea, and this quote Who beat you ?
It was Nick
He ask him even if he sees the result - he wouldn't ask this quetion if he didn't see the any injures.

Yea but I wonder which one I should I use.

Thank you for correction, but it didn't answer my question.

Here is an interesting discussion on the subject.
forum.wordreference.com/threads/i-have-forgotten-or-forgot-in-this-case.2753641/