What is the climax, falling action, and resolution of "The Fall of the House of Usher"?
The climax of "The Fall of the House of Usher " could be said to occur when Roderick tells the narrator "We have put her living in the tomb!" just before Madeline reappears. In the days leading up to this crisis, Roderick's instability has mounted and Madeline's health has declined to the state of her becoming moribund, and the narrator has just become unnerved by mysterious and ominous sounds coming from outside the chamber where he and Roderick are reading together.
The falling action could be said to occur as the narrator describes Roderick falling to the floor dead in his sister's fatal embrace and his own flight from the House of Usher. As he is fleeing from the horrific scene he glances back and sees the blood red moon clearly through the wide-open fissure dividing the house as it collapses.
The brief conclusion of the story is the narrator's description of the waters of the tarn closing atop the splintered remains of the house and its inhabitants.
I think that the climax is the night of the eight day, when they start to hear noises and when Madeline bursts into the room where he and Roderick are. The falling action is the ensuing struggle and when both Roderick and Madeline are dead. The resolution is when the narrator runs out of the house, and he then sees the house crumble.
However because of the way this story is structured, you could adjust it somewhat.
(Just to add to the answer above) The climax is when Roderick and the narrator realize that they buried Madeline alive and when she falls onto Roderick.