Biff's sudden realization that his father is having an affair has a devastating effect on him. He previously looked up to and idealized Willy, and suddenly, in a vulnerable period of late adolescence, he realizes his father is a badly-flawed human being who has deceived his wife and family merely to indulge in his own pleasure. Biff loses faith in his father and his father's advice.
This crisis comes at critical moment in Biff's life. He was supposed to attend summer school so he could graduate from high school. (This play is set in the Great Depression, at a time when many Americans did not finish high school, so this graduation would have been an important achievement.) Crushed by his father's betrayal and angry at him, Biff refuses to attend summer school.
As is typical of this family, Biff doesn't talk about what happened but instead buries it away, once again allowing his father to live with his self-destructive illusions.
Already at this point fragile, Willy is confused and...
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