In The Great Gatsby, how does Nick's relationship with the past contribute to the meaning of the novel as a whole?
In the first chapter, when Nick talks about his life before the war compared with his life after it, we get a sense that life was dramatically different then than it is now, after the war. Nick says that he "came back restless. Instead of being the warm center of the world, the Middle West now seemed like the ragged edge of the universe [...]." To paraphrase the old adage, it really isn't possible to go home again. In a way, Nick has lost his innocence; he's seen and done too much to be able to return to his former life and be his younger self, the self that thought home was everything. He has realized that we can't turn back time, no matter how much we may want to, and so we must move on.
However, this is a lesson that Gatsby never seems to learn. When Nick tells him that he cannot repeat the past, Gatsby cries, "'Can't repeat the past? [...] Why of course you can!'" Gatsby continues to believe that no matter how much has changed, he and Daisy can still be together; never mind that she is now someone else's wife and a mother. No matter that she has moved on because he has not. Perhaps, if he'd been able to recognize that we can't turn back time or repeat the past, he wouldn't have continued to pursue Daisy, ran afoul of George Wilson, and ended up dead.
Therefore, between Nick's recognition that one cannot return to the way life used to be and Gatsby's failure to understand this, we learn that one of the novel's most important themes is the impossibility of returning to a state of innocence or stopping the passage of time.