What does Hemingway have in mind when he talks about "one perfect sentence" in this novel?
This is three questions, and you only get one. I'll address the third, and you'll have to find what you think might be the perfect sentence according to Hemingway.
You're not looking for one right or correct sentence, as there is not just one in this novella. Instead, Hemingway believed generally that shorter was more effective than longer and that one perfect word was better than many mediocre words. By perfect, I mean in context and usage, a word which says exactly the thing you intended it to say. For example, explaining something as a large, gray, wrinkled animal is not stronger than simply calling it an elephant. Many words and descriptors do not compete, in Hemingway's view, with a simple, accurate statement. That doesn't mean he's boring or "plain"; it means he has chosen a more streamlined approach to writing. His sentences are much shorter than most writers'. In fact, you won't see any punctuation much more complicated than a comma in any of his work. Simple and direct while still painting a picture for his audience--that was Hemingway's approach to writing. The Old Man and the Sea is not a long work; look through it again and see if you can find some of what I mean in this writing. If you're still struggling, look particularly at the descriptive passages of the sea or the marlin or Santiago's dreams. These are all places where Hemingway evokes simple yet effective imagery in his uncomplicated style.