In the "The Open Window," if the window had been closed, how might the story have been affected?
If the window had been closed in the story "The Open Window," the most poignant moment of the story would have not occurred with as much intensity as it would have in any another way. This is not to say that the actual event would have never taken place. It just may have happened in a less climactic form.
First, Vera would have needed to find a different focal point for her story. After all, it is she who initiates the action by asking Framton Nuttel:
"You may wonder why we keep that window wide open on an October afternoon," said the niece, indicating a large French window that opened on to a lawn.
Since Vera is so versed in coming up with last-minute stories, she uses the open window as her starting and focal point. If the window had been closed, she would have had to use a door that would knock or a bell that would ring upon the arrival of the alleged dead men.
Now that she has Framton's attention, the window will serve as a picture frame. Framton will now be eagerly looking at the window in a way much similar to how we watch a movie, waiting eagerly for something to happen. These dynamics between Vera and Framton would have been lost if the window had been closed.
Vera also points out that, "out through that window" three years ago, the husband and brothers of her aunt "went off for their day's shooting." If the window had been closed, she could have not pointed at it as the exact origin from which the men "went off shooting." Keep in mind that Framton is still focused on the window as well, particularly after Vera creepily informs him that:
....sometimes on still, quiet evenings like this, I almost get a creepy feeling that they will all walk in through that window...
The fact that the window is open provides that opportunity for anything to happen: People can creep in. An animal may jump in. Someone may jump out. The openness of the window creates more possibilities.
These possibilities would have not occurred if the window had been closed. The anticlimactic effect of a closed window, or a typical knock at a door would have still made Framton break down, but the story would have lacked the anxiety and built-up energy caused by whatever may be seen through the open window.
Also, since the window is open, and Vera knows that the men come through it after they have gone hunting, something will definitely be happening in the house that day. She knows what will take place precisely because the window needs to be opened during hunting days. She knows that her trick will work. This is why the window is so essential to the story.