Imagine the following scenario:
I read a novel two years ago. Today I cannot remember the story of the novel any more, I've forgotten the story. Somebody asks me:
Have you read the novel?
What should be my grammatical correct and natural sounding answer?
Yes, I've read it but I've forgotten the story.
Yes, I had read it but I've forgotten the story.
No, I had read it but I've forgotten the story.
No, I read it but I've forgotten the story.
My uncertainty is due to the grammar explanations that Present Perfect indicates a state in the present. According to my Grammar book the following text is correct:
Tom is looking for his key.
He has lost his key.
This means that he doesn't have it now.
Ten minutes later: < picture of Tom holding the key is his hand >
Now Tom has found his key.
He has it in his hand now.
Has he lost his key?
No, he has found it.
Did he lose his key?
Yes, he did.
He lost his key but now he has found it.
The present state of having read a novel is to know its story. But if after having read the novel I have forgotten its story then I am no more in this state although I was in this state before because I read the novel in the past.