"Bent double, like old beggars under sacks, Knock-kneed, coughing like hags, we cursed through sludge," (1-2)
The bodies of the soldier are twisted and contorted, making their experience seem completely different from the sorts of marching that we usually see in military parades. Here they're like "beggars" and "hags" – cast-off elements of society.
Men marched asleep. (5)
Ok, maybe sleeping is the best thing that people can do in the midst of all this trudging through mud and bullets - but sleep deprivation can't be all that pleasant. Unfortunately, in this poem, it's the least of the speaker's worries.
"Many had lost their boots But limped on, blood-shod. All went lame; all blind; Drunk with fatigue; deaf even to the hoots Of tired, outstripped Five-Nines that dropped behind." (5-8)
Words like "lame," "blind," "drunk" and "deaf" suggest that the soldiers have been stripped of their bodily integrity before they even enter into battle. They're almost zombie-men, stumbling through the dark with bodies that don't work anymore. And that's before the gas attack.
"But someone still was yelling out and stumbling And flound'ring like a man in fire or lime..." (11-12)
Lime, or quicklime, is a chemical compound that can burn through the human body (sort of like fire). Referring to death by fire or lime allows Owen to describe the horrors of gassing as both natural and unnatural suffering…it's like fire and lime-burns combined.
"In all my dreams, before my helpless sight, He plunges at me, guttering, choking, drowning." (15-16)
Because the trio of verbs in line 17 are gerunds (verb forms that end in –ing), we get the sense that the action is in the present tense. The speaker's comrade dies over and over in his dream, making the suffering of wartime casualties never-ending.
"…the white eyes writhing in his face, His hanging face, like a devil's sick of sin;" (17-18)
The man described here seems almost inhuman…as if the physical effects of gassing can transform his body into a version of hell on earth. His very face begins to melt off of him.
"If you could hear, at every jolt, the blood Come gargling from the froth-corrupted lungs, Obscene as cancer, bitter as the cud Of vile, incurable sores on innocent tongues,—" (21-24)
The intense imagery of these lines emphasizes how unimaginable such horrors are for the civilian population. No one can understand how excruciating it is to die of gas poisoning, unless, of course, you're watching your comrades choke on their own blood.