into Farquhar’s head as last-minute thoughts of his wife, children, and possible escape develop parallel to external sensations, like his ticking watch. Part I functions asexposition that transitions into part II.

In part II, the narrative shifts to a time in the past when Farquhar and his wife encountered a person they thought was a Confederate soldier. Farquhar becomes more than merely the condemned man of part I. He is given a background (a wealthy slave owner who could not join the Confederate army) and shown to be eager to help out the Confederate cause in any way. Chafing under whatever restriction prevented him from joining the military, he jumps at a chance for glory and distinction. Therefore, he becomes easily duped by a Federal scout into trying to burn the northern military’s stockade at Owl Creek Bridge. Part II thus functions as the ironic backstory to Farquhar's hanging.

Part III brings readers back to the present, when Farquhar descends into water as he is hanged from the bridge. The rope seems to have broken and dumped his body into the stream. An omniscient narrator describes Farquhar’s thoughts as he sinks. Farquhar feels physical pain and near-drowning sensations, but then shoots back up to the surface, evades bullets, and flees back to his home. The story ends with Farquhar reaching out to his wife, but the narration suddenly returns to the removed, third-person observation of Farquhar’s dangling body. Part III presents Farquhar imagined escape; this section functions to demonstrate that his escape fantasy was as far from reality as his belief in the words of the Federal scout.

The three sections of "An Occurrence at Owl Creek Bridge" differ in their points of view: the first section is in Omniscient point of view, the second is in Objective point of view, and the third in Third-person Limited which includes stream-of-consciousness.

1. Omniscient point of view

In this section the Realistic elements of Bierce's writing are evinced. With great detail the depiction of the military protocol adds a certain impact as it denotes a tenor that is richly suggestive to the reader with the repetition of silence. For instance, the description of the execution details this silence:

The company faced the bridge, staring stonily, motionless. The sentinels, facing the banks of the stream, might have been statues to adorn the bridge. The captains stood...silent, observing the work of his subordinates, but making no sign.  Death is a dignitary who when he comes announced is to be received with formal manifestations of respect even by those most familiar with him. In the code of military etiquette, silence and fixity are forms of deference. 

The solemnity of the protocol of the military in deference to Death also helps to magnify the irony that prevails throughout Bierce's narrative as Farquhar's death is a less than glorious condition of war.

2. Objective point of view

The move from Omniscient narrator to Third-person Objective narrator enhances the sense of reality in the story as there is a detailed report on Farquhar and his background and motivation, as well as the circumstances which led to his capture. This narration also refines the element of irony as in the last paragraph the reader learns that the soldier to whom the Farquhar's extended their courtesy is, in fact, a Federal scout.

3. Third Person Limited point of view

From the realistic details of the first two sections, the third section moves to the random thoughts of stream of consciousness.  Time is slowed down and there is an intricate fantasy woven with Farquhar's reverie; in fact, this intricacy plunges the reader into the figments of Farquhar's thoughts, a condition which certainly produces the shock of the brutal, matter-of-fact ending, It also magnifies the irony of Farquhar's name--Gaelic for manly, or brave--along with the delusions of the glorious condition of war.

In his critical essay, "Something Uncanny: The Dream Structure in 'An Occurrence at Owl Creek Bridge,'" Peter Stolchaff, while acknowledging the clever change of narrators, writes of Bierce's other purposes:

More significantly for Bierce's purposes, though, is that "time" itself, when employed to calibrate human experience, seems to become indeterminate at points of maximum emotional disturbance.

In the first section, there is the silence that seems indeterminate, as well as Farquhar's glance at the driftwood that appears to move slowly.  In the second section, there is a hint of a desultory life as Farquhar and his wife sit one evening on a hickory bench. Finally, in the third section, the reader is placed inside the consciousness of the main character.  There is, indeed, a dreamlike structure that develops and reaches its crescendo at the end.

If you're after certain structural elements instead of simple partitions of the story, consider these -

Use of the third person limited point of view -  The story takes place from the viewpoint and understanding of the protagonist only.  Most of what happens is only in his mind, as the reader learns at the end of the story.

Use of flashbackand virtual reality - Although the reader is not aware of it at first, the "events" in the story don't happen in chronological order but are subjectively selected (between real and imaginary ones) by the character of Farquhar.

Use of dramatic irony and a surprise ending - Up until the very end of the story, the reader "lives" the events of Farquhar's fantastic escape. Just as he "makes it," though, the reader realizes it has all been an illusion as Farquhar's neck snaps under the tension of the rope.

Your question is a bit unclear, but I assume you are referring to the main elements of the three parts of the Ambrose Bierce short story, "An Occurrence at Owl Creek Bridge."

In Part I, Peyton Farquar is about to be hanged on the railroad bridge over Owl Creek. He sees everything more clearly and in slow motion as his execution nears. As he thinks about his home and family, the captain signals the execution to be carried out.

Part II is exposition, a flashback that recounts Farquar's past and the actions that have put him in the hangman's noose. He has planned to burn the Owl Creek Bridge, but he confides in a Southern soldier who is secretly a Northern scout--a spy.

Farquar escapes the noose at the beginning of Part III: The rope has snapped and he tumbles into the water below. He struggles to break free of his restraints and rises to the surface, only to be met by bullets and cannon fire. But he escapes, swims to safety and heads back home to see his wife and kids. At last, he sees his house and reaches out to hug his wife... Or does he?

The final sentence explains the truth:

Peyton Fahrquhar was dead; his body, with a broken neck, swung gently from side to side beneath the timbers of the Owl Creek bridge.

The use of deception--both on the part of Farquar and the Northern spy as well as the author's creative way of misleading the reader--is a major theme. Farquar's manipulation of time is another focus of the story. The relationship of death vs. reality is yet another important motif that makes this one of the finest of all American short stories.