One of the remarkable things about Arthur Miller's play is the way he has managed to show past and present as well as reality and subjectivity all in a single stage set. Most plays and movies are in the present tense. What we see onstage or on the screen is happening now. The scripts are even written in the present tense, unlike most novels and short stories. Movies have conditioned audiences to understanding some cinematic "vocabulary," including the meaning of "flashbacks." An actor is gazing out the window or into a pool of water and then the camera creates a "slow dissolve" or an "oil dissolve" and we know we are back in the past--but the past is still rendered as the present. It is hard to escape from the present tense in drama. (An interesting French film that tries hard to escape the iron grip of the present tense is Last Year at Marienbad.) Arthur Miller has created an impressionistic set in which different locations represent the present and the past. The audience is to understand that whatever is supposed to be taking place in the past is occurring as the present in Willy's "imaginings." The scenes that take place in what Miller calls the "city scenes," which occur in the present tense in the same part of the stage setting. In the case of the city scenes, they are occurring in the past but in a different location away from the Loman home. It would seem that Miller intended almost everything that occurs on the forestage to be taking place in Willy's "imaginings." Miller depicts past and present, reality and subjectivity. Here is an excerpt from his detailed description of this single impressionistic stage setting.
Before the house lies an apron, curving beyond the forestage into the orchestra. This forward area serves as the back yard as well as the locale of all Willy's imaginings and of his city scenes. Whenever the action is in the present the actors observe the imaginary wall-lines, entering the house only through its door at the left. But in the scenes of the past these boundaries are broken, and characters enter or leave a room by stepping "through" a wall onto the forestage.
Willy's encounters with his brother Ben would take place on the forestage, as would his meeting with Howard. So too would his memories of the days when Biff was a high-school football hero. The audience quickly catches on to the avant-garde stagecraft devices and succumbs to the illusions of being in the past, present, or inside Willy's imagination in a sort of never-never land.
For the sake of comparison, Shakespeare never tried to show the past as the present. He would simply have one of the characters describe a past event in dialogue. Miller's play is a good example of modernism in theater because of its impressionistic staging as well as its intentional and radical flouting of Aristotle's unities and the ancient Greek philosopher's dictum that a tragedy had to deal with the downfall of a Very Important Person.