Willy is mentally collapsing at this point. He had difficulty distinguishing between the past and present earlier in the play, but the possibility of things getting better still existed. By Scene 9, Willy knows that all is lost — both his job and Biff's chance of success — so he resorts to the past to escape the present. Biff's failure with Oliver immediately moves Willy back to his son's failure in high school. As Biff tries to explain what happened with Oliver, Willy is caught in the past, still trying to understand what it is that caused Biff to "lay down" in high school and how that connects to his failure today. Willy is desperately trying to regain order in the present by making sense of the past.
Learning that Biff stole Oliver's pen temporarily brings Willy out of the past. Willy feels responsible for Biff's actions, and he immediately moves back into the past to find justification for the theft. Biff states, "I didn't exactly steal it [the pen]!" but it is impossible for Willy or the audience to believe this based on his previous record that includes stealing the football, as well as the building materials. Willy is partially to blame for Biff's actions simply because he sanctioned his behavior every time before by not making Biff face the consequences. Therefore, because Willy taught Biff that he did not have to follow rules in high school, his behavior in the present is a reflection of his previous conditioning. As a result, Willy bears the primary responsibility for Biff's present failure.
