For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
The narrator slowly exposes himself to us, tries not to fall in love, gets his leg torn up while eating cheese and waiting to help the men who are about to be wounded, and then does fall in love, and helps make a baby.
Act II
Frederic gets sent back to the front for being a smart aleck, and then flees from the army to save his life by swimming up a roaring river. He finds his Catherine, and they escape into neutral Switzerland and live the good life.
Act III
But when Catherine goes into labor, the gig is up. First their love-child dies, and then so does Catherine. And Frederic walks off into the rain alone.