What is the formal structure of "Dulce et Decorum Est" by Wilfred Owen?

The poem "Dulce et Decorum Est" by World War I poet Wilfred Owen does not adhere to any sort of formal poetic structure. Its four-stanza structure is irregular, as the first stanza contains 8 lines, the second stanza 6 lines, the third stanza 2 lines, and the final stanza 12 lines. A rhyme scheme does exist, but it too is irregular, overlapping stanzas in a rhythmic yet slightly unpredictable manner.

The irregularity of the poem's structure contributes greatly to the experience of reading the poem. It is disorienting to read, much like the mustard gas that is the central image of the image was disorienting to the men who suffered from the effects of the poison during the war. As well, the imagery of the drowning man, helplessly grasping at anything, grabs hold of the reader unexpectedly, in his own 2-line stanza, highlighting the plight of the individual in this war that killed en masse.

Though the poem has no formal structure, that is not to say that the irregular structure that can be identified is meaningless. In fact, the irregularity is part of the point and the experience that the poet desires to communicate to his readers.

This poem can also be viewed as two sonnets. The first two stanzas, one of eight lines, an octave, and one of six, a sestet,could be seen loosely as a Petrarchan (Italian) sonnet though not in classical form as Owen does not strictly adhere to that rhyme scheme. The second portion of the poem is similar to a Shakespearean (English) sonnet, which consists of three quatrains (four line stanzas) and one couplet (two line stanza),though the stanzas are not broken into visible quatrains, but instead two stanzas, one of two lines and one of twelve lines.This may be one interesting way to look at this poem's structure.

There is no formal structure for this poem.  It is a 28-line poem that does contain some rhyming; the rhyme scheme is one that follows the pattern of abab cdcd efef, etc.; however, they are not presented in quatrains. The lines are broken up. There is more useful information at these two links:

http://www.enotes.com/dulce-et-decorum-est-salem/dulce-et-decorum-est-9650000186

http://answers.google.com/answers/threadview?id=210188

What is the structure of "Dulce et Decorum Est" and which structural techniques are notable?

The central stanzas, detailing the loss of a man to gas:

GAS! Gas! Quick, boys!-- An ecstasy of fumbling,
Fitting the clumsy helmets just in time;
But someone still was yelling out and stumbling
And floundering like a man in fire or lime.--
Dim, through the misty panes and thick green light
As under a green sea, I saw him drowning.

In all my dreams, before my helpless sight,
He plunges at me, guttering, choking, drowning.

--are particularly effective in their repetition of themes; "fumbling, clumsy, stumbling, floundering, guttering, drowning." These words, showing the desperation of the affected man and his plea for help, allow the intimate and immediate understanding of toxic gas and its horrible effects on the body. The narrator can barely see the dying man "under a green sea," but remembers him vividly in his dreams nonetheless. 

Crucial to the structure of this poem is the exceptionally long dependent clause of the final stanza. We don't reach the subject -- "My friend" (said with sarcasm) until the poem is nearly over, and then the final lines of the poem are quick and stabbing, with the cliched advice saved until the very end, where it seems exceptionally ironic.

As mentioned, there is a varying length of different lines. There are stanzas that also vary: the first stanza is eight lines, the second is six lines, followed by a couplet that rhymes with the last two lines of the six-line stanza that precedes it. The last stanza has twelve lines. 

Wilfred Owen's poetic structure reflects the topic of the stanza. While the eight-line stanza, he describes soldiers trudging. The next, shorter stanza may well reflect the pace of the men as they are warned of the danger of the gas. (These images of WWI are expertly captured by Owen.)

The couplet reflects the haunting scene of this moment, that lingers—and Owen sets it apart from the reality of that moment, suspended as the dream that lingers in Owen's consciousness.

The length of the final stanza is much longer, and the movement slower—mimicking a funeral march—as the soldier is carried in the wagon, dying slowly. There is the sense of the unreal, like nightmares (as Owen points out) as they wagon moves. Owen uses the structure of his poem to reflect the message in the poem. 

The uneven stanza length can be said to represent the unstable and unpredictable events of the poem. Owen is clearly able to use meter and rhyme skilfully, but plays with these techniques by giving us repetition -'drowning' as we have come to expect rhyme.

The use of the Homer phrase in Latin gives a finality that the deaths are a cruel tradition cloaked in respetability and civility. By qualifying the statement as a 'lie' Owen forcefully asserts his anger at the pointless deaths.

The two-line stanza is separated from the rest of the poem's structure in such a way as to draw attention to the suffering of the young man who is dying from the gas attack. I also think the "we-I-you" progression is crucial, as this is a polemical poem. Owen also deliberately uses very harsh language to convey his complete and total disdain for those who would glorify death in war.

The "story" of each stanza is also something to notice. The poem starts with a stanza about war-weary soldiers and it is sad enough to read, but the second stanza brings the true problem of the war -- the mustard gas that the enemy is using. This stanza brings a lot of very active imagery to the fore as the men are scrambling after the attack. The short two-line stanza is the climax of the story as the soldier stumbles and chokes to death. The last, long stanza is the immediate response to this man's death and then extends out to the theme of the poem as a whole: that dying in war is a glory is really just a huge lie.

Key to focus on is the switch of stanzas and how they contain varying line lengths. Also note the way that the poem begins by using the first person plural, "we," then shifts to the much more intimate "I" before going on to use the accusatory "you" as the speaker seeks to involve the reader in what has been described and to make them take a side in the issue of warfare.