4 ideas
Through a lighted window he sees her laughing among German soldiers, and he leaves unseen without speaking to her. Back in the trenches he talks himself round, reasoning that people cannot grieve without pause and that life at home carries on without the absent soldier. He is later killed by a shell.
Soldiers return from leave or hospital and find that the rear, meaning shirkers, clerks and civilians, talks of the war in heroic abstractions while living comfortably from it. Direct suffering cannot be passed on through talk, so a society at war splits into those who bear the cost and those who narrate it. Each side's picture of the war becomes unrecognizable to the other.
In the flooded trenches at the end, the surviving soldiers conclude that the men facing them are laborers and peasants just like themselves. They blame the war on a small class of rulers, profiteers and militarists who benefit from it and stay safe. They argue war will end only when the common people of every nation see this shared position and refuse to be set against each other.
A soldier asks the narrator whether he will keep their swearing and slang when he writes the book, and the narrator says he will, because removing it would be a lie. Honest witness means keeping the ugly, ordinary texture of experience, not raising it into polite or noble language. Tidying up the speech of the people who suffered is a quiet form of falsification.