the novel begins with a flashback, what do you think is its purpose? what do you learn about the narrator?

The purpose of flashbacks are usually for the authour to give the reader certain information to carry through the plot of the story. Before we know anything about the protagonist, including his name, we learn that one moment in his past has defined his entire life. We do not learn exactly what the moment is until Chapter Seven. This tells us that the event has significance beyond its detail; it is not so much specifically the rape, but more generally the betrayal, that makes that moment in time so central to Amir's life. The rest of the novel is, more or less, built around this flashback incident.