The narrative structure of American Pastoral is rather complicated. To begin with, the novel's main plot line—the story about the Swede—is a frame story told by the character-narrator Nathan Zuckerman. In addition, this story is a sort of imagined flashback with additional flashbacks woven into it. Moreover, because the narrator is a famous author who reveals from the beginning that the rest of the story is a result of his own fabulation, the narration is also inherently unreliable.
In the first chapter, the narrator's position and the narrative structure feel pretty straightforward. The narrator's first-person experiences occupy a relatively central part of this chapter.He writes in the first person, and, while his main focus is the Swede and the Levov family, details about his own life play an important role in the story he's telling. Although he includes a number of flashbacks in his narration, the temporal situation is also quite clear. Looking back on his early years, the narrator is speaking to the reader from somewhere between middle and old age.
Over the course of Part 1, however, the narrative structure becomes much more muddled. Chapter 2 opens with a speech Nathan Zuckerman writes after his high school reunion—in which he looks back on the same period he already dipped into in Chapter 1, this time from the more general perspective of his graduating class. After this, he rewinds slightly and begins to tell the story of the reunion that precipitated the speech. Chapter 3 begins when, at the reunion, he meets the Swede's brother Jerry Levov and discovers not only that the Swede has just died but also that Zuckerman had completely misunderstood him. This revelation sparks the project that turns into the rest of the novel: to tell the story of "the Swede's great fall."
Over the course of Chapter 3, Zuckerman's first-person point of view is gradually displaced. When the narrator says that he "found" the Swede in Deal, New Jersey "the summer his daughter was eleven," this is pretty much the last time Zuckerman expresses himself in the first person in the rest of the novel. From this point onward, the temporal situation toggles back and forth between the years before and after Merry's disappearance. Lacking a conventional chronological structure, the narrative in large part consists of a long and associative thread of flashbacks.
The high school reunion thus acts as a framing device, from which Zuckerman looks back on the life story the Swede never got the chance to tell him. It's worth noting that, almost right away, Zuckerman acknowledges that he's an unreliable narrator. In no way claiming that this is how the Swede's life really unfolded, he instead describes his project as "[dreaming] a realistic chronicle." Roth thus seems to be exploring and commenting on the act of storytelling and the notion of truth. Relatedly, it's hard to avoid being struck by the self-referentiality of Zuckerman's project, in which he forces himself to "inhabit this person least like myself, disappear into him, day and night try to take the measure of a person of apparent blankness and innocence and simplicity, chart his collapse, make of him, as time wore on, the most important figure in my life." As Roth makes up the character and life of Zuckerman, Zuckerman makes up the character and life of the Swede.