"Bent double, like old beggars under sacks, Knock-kneed, coughing like hags, we cursed through sludge," (1-2)
Using the word "like" to create two similes (the soldiers are "like" "old beggars" and "hags"), Owen suggests that their reality is so surreal that he needs to find comparative ways to describe it so that his readers can understand how gruesome his experience has been.
"Dim, through the misty panes and thick green light, As under a green sea, I saw him drowning." (13-14)
The many veils between the speaker and the dying man create an otherworldly sense in these lines, almost as if the man is already in the underworld (or hell).
"In all my dreams, before my helpless sight, He plunges at me, guttering, choking, drowning." (15-16)
After the events narrated in the poem, the speaker's dreams are as real (if not more real) than his waking experiences. The war becomes a mental battle, one which doesn't stop wrecking his mind even after the official fighting has ceased.
"If in some smothering dreams you too could pace Behind the wagon that we flung him in, And watch the white eyes writhing in his face, His hanging face, like a devil's sick of sin; 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,—" (17-24)
Starting this stanza with "if" allows the speaker to subtly point out the distance between himself and his readers. We don't have smothering dreams. We can't hear the dying soldier's gasping breath. When we read the poem, we only experience these scenes as conditional descriptions. "If" we could see them, we might understand – but for the speaker, that seems to be a pretty big "if."
"My friend, you would not tell with such high zest To children ardent for some desperate glory, The old Lie:" (25-27)
Referring to the reader (or his intended audience) as a "friend" seems like an ironic move on the speaker's part. After all, it's the enthusiastic ignorance of the "friends" participating in war efforts at home which got the speaker into this horrible mess in the first place.
"…Dulce et decorum est Pro patria mori." (27-28)
Why end with a quote in Latin? Well, for one thing, it's a direct quote from Horace. For another, it emphasizes the foreignness of such concepts as "patriotism" and "glory for one's country" on the battlefield. Once you get into the war, the speaker suggests, such words are nothing more than a dead language.