Biff’s diary would probably focus on his feelings of being trapped, and his father’s betrayal of him.
Biff is frustrated by his father. He seems to consider his actions a betrayal. He complains to Linda.
Because I know he’s a fake and he doesn’t like anybody around who knows!
Biff does not want to share with her about his father’s affair. In his diary entry, he would most likely talk about his desire to move out of his father’s shadow. His father thinks he is lazy. Biff thinks marrying and moving out of the house, away from his father, will prove he is not lazy and get him away from his father’s negative influence.
Yet Biff is torn between wanting to break away, and his ties to his family.
I hate this city and I’ll stay here. Now what do you want?
His mother tells him his father is trying to kill himself. Biff still feels betrayed. He feels like his father is constantly manipulating him.