I have a problem.
I don't understand what does this "result" should look like in present perfect, because when I see in movie that a character says: I forgot it. The result is that he doesn't remember it or maybe he didn't do something. So how should I interpret it?
1a) "Aaaaaah! I forgot to buy something for my friend"
Interpretation: normal conversation between two characters (my interpretation is that she use "PAST" simple because she wanted to give an information, even if the result is that she doesn't have this thing)
1b) "Aaaaah! I've forgotten to buy something for my friend"
The result is that he doesn't have this thing he had to buy for his friend
2a) "Hey Tom. There was an apple in the fridge, what happened to it?"
Tom: "Yea I ate it."
Interpretation: Even if the apple wasn't in fridge "result" we are informing him, or I guess it is supposed to be some kind of informal thing. Or maybe this is some kind of emphasis?
2b) "Hey, Tom. There was an apple in the fridge, what has happened to it?
I've eaten it"
Well, he ate this apple and the result is that there is no apple in the fridge.
3a) "I haven't delivered the package."
The result is that he is holding that package, or maybe some kind of info.
3b) "I didn't deliver the package."
He just inform that he didn't deliver the package, maybe he hold this package but we don't care about result, we care about if he did it in past or not.