How does the past impinge on the present in a tragic manner in Death of a Salesman, by Arthur Miller.

In Death of a Salesman the past continuously impinges upon the present. This is because the entire play is based on Willy Loman's final reckoning; the last episode of his long life of mistakes, where he finally comes to terms with what he has done, and makes his first (and last)  attempt to redeem himself. 

Willy's life story comes to us as flashbacks. Since Willy is a sickly, tired, aging man, these flashbacks from life serve as a way to tell the audience that life is, literally, "flashing in front of his eyes". This concept is often attributed to an imminent death, which is what ends up happening in the end. 

Aside from presenting itself in the form of flashbacks, the past has also made a massive dent in Willy's life. It is because of his past that Willy's life is as empty, shallow, unfruitful, and inconsequential as it is now. 

This is why.

Willy had choices that he did not want to make. He had many opportunities to join his brother, Ben, and go get the riches that Ben had acquired by going to the mines. Willy, ever second guessing himself, chose to listen to his wife Linda and decline the invitation. However, now in his twilight years, this indecision comes back to haunt him. What if he had taken the risk? Would he be as hard for money as he is now, in his sixties? Would life have been easier?

Willy's bad choice-making also comes in the form of the career choice that he picks. Willy does not like selling; otherwise he would be good at it. He claims to be a good salesman, but he is probably quite average. Regardless, the only reason why Willy becomes a salesman is to follow the steps of another salesman named Dave Singleman, a myth of a man whose funeral was attended by everyone.

The problem is that this is like chasing a waterfall: no one must plan a life based on the life of someone else. The path of each individual must be walked alone. Willy's choice to follow the life of Singleman leaves him in a job in which he does not succeed nor excel. He does not even leave anything for his family to reap any benefits from. 

The past also comes to haunt the present in the form of Biff's unproductive life. His once popular and successful life as a potential football player comes crashing down when he accidentally discovers that Willy had a mistress. While Willy continuously blocked this incident, and blamed Biff for his own problems, it is clear that this crass mistake on Willy's part definitely ended up hurting the entire family, altogether. 

In all, Willy's past demons come to haunt him now that he is in his very last days of life. Perhaps these flashback serve as one last attempt to ask Willy to come out and redeem himself before the end.