Continuing with structure, there are a few contrasts in this act, which I will point out. Firstly there is a big contrast between what he tells his sons about how well he is doing and the real truth which is he is not being successful. He says to Biff and Happy "Knocked 'em dead in Providence" but he says to his wife "People don't take to me". The main contrast in this scene is when he's talking to his wife but then starts talking to the woman he has an affair with.
When Willy is talking to Biff and happy Arthur Miller creates the mood of happiness. This is because Willy is looking back into the past and he thinks that the past was a better time for him. Arthur Miller does this by changing the scenery from buildings to trees giving the impression of time when there were no buildings i.e. in the past. Also we hear a solo flute being played which connects him to the memory of his father whom played the flute. This also suggests a happier feeling on the part of Willy. Arthur Miller also makes the lights brighter making the act not seem normal and therefore what Willy is thinking. Brightness is connected with happiness, which also sets the tone for this scene. What Willy tells his sons is harmful because he lives in an unrealistic, fantasy world. He tells them that personality is important and that you don't have to work hard to get far. He makes himself seem successful and well liked but he later tells Linda that this is not true. This theory is very much wrong because all the hardworking people who Willy claims are not well liked are doing better in life than he is for example his brother Charlie. This is partly why his sons fail in life because they believed that life was easy and they didn't have to work hard- when they did try to do anything, they overestimated themselves and didn't succeed. Arthur Miller includes Bernard in this play to cast doubt on Willy's happy memories. He is also used to prove Willy's theory wrong because he works hard and is not the most popular person ever but is still successful however.
The scene where Willy talks to Linda suggests that Linda is not fooled by Willy's story that he has been really successful on business. Instead she gradually investigates by adding up the figures and works out that they don't match with Willy's story. This is where Willy confesses that, well things didn't go that well and he's not happy about it. This makes Linda seem more understanding and less harsh than if she had said something like "that can't be true".
The scene where we see Willy talking to the woman he has the affair with uses a number of normal props. When Willy is talking to his wife, she laughs and blends in with another woman's laugh. The light then is shown on the woman. Arthur Miller wants to create the reality that this is all happening in Willy's head. Therefore he needs to make the woman magically appear. He does this by leaving an area of stage on the far left darkened where nothing can be seen. The woman is behind a scrim, a scrim only allows light to go through one way. So before she appears the light is shone at an angle but to make her appear the light is shone straight through and she can be seen clearly. This gives the impression of memories appearing in Willy's mind. Willy gives the woman a pair of stockings and afterwards she disappears. We again see Linda mending a pair of stockings but Willy says, "I won't have you mending stockings in this house". This points to Willy's feeling of shame because he's just given brand new stockings to a woman he's having an affair while his hardworking and faithful wife has to do with old broken ones. I think this provokes sympathy for Willy from the audience because Willy knows he's done an extremely bad thing and is very sorry about it but he can't get out of the back of his mind.
The end of this scene is the climax of the crescendo of Willy's guilt. It started with Willy liking the past when he was successful and his kids had a good future but as he remembers all of the bad things they all pile up and he 'explodes' saying "There's nothing the matter with him!" Willy realizes at the end that it was a really bad time and that things weren't as good as they seemed.
Conclusion
Arthur Miller doesn't just use the flashback as one of Willy's memories but to show Willy's state of mind and that he liked the past better than the present but finds cracks when recalling some of it. It also explains why the present is the way it is because Willy sometimes misled them. The structure helps us to understand it because it shows us how Willy's shame escalated when he realizes all the bad things he did.