What role does Biff's discovery of Willy's infidelity play in the tragedy that has befallen the Loman family in Death of a Salesman?
Biff's discovery is essentially the catalyst for the events that ultimately bring about Biff and Happy's failures and Willy's death. When Biff discovers Willy’s affair, it becomes clear that Biff’s anger with Willy over the last 17 years stems from his knowledge of Willy’s dishonesty. Rather than reveal Willy’s infidelity to anyone, Biff has remained silent and held a grudge against his father. Willy too has suffered with his guilt, taking his anger and shame out on the son who knows his secret. At one point, Willy throws Biff out of the house because Biff knew he was a “fake.” Although Biff never explained his reasons for calling his father a fake and phony, we now recognize Willy’s affair as the source of Biff’s hostility.
Of course, Biff’s discovery affects every aspect of his life form that point on. After discovering the affair, Biff dismisses his father’s chances of convincing the math teacher to let Biff graduate. The trip to visit his father turns into a life-shattering moment. Biff transforms from a loyal son who idolizes his father (seeing him as the symbol of self-confidence needed to achieve the American dream) into a son betrayed by the father's emptiness and selfishness. Biff probably did, as Willy has suspected, decide not to attend summer school and graduate as a way of hurting, or spiting, Willy. This spirals into a life of failure and drifting, reflecting Willy's own failures in business, and the tragedy of his aging. Despite all this, Willy accuses Biff of spiting him rather than taking responsibility for his own actions and their effects on his young son when he visited him in Boston.