How we cite our quotes: line
Quote #4
"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."
Quote #5
"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.
Quote #6
"…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.