The motif of secrets
The motif of secrets starts early. The premise is that an old woman is finally sharing a lifelong secret. The girls keep secrets from each other as a kind of game. They share other secrets in silence. Ultimately, the climax of the novel is the revelation of another secret—the secret suffering of veterans with PTSD.
Robbie's story
People who haven't gone to war before don't understand what it's like to be in a battle for one's own life. But Robbie knows that terror firsthand, and he knows that humans are capable of greater forms of evil than anyone in his life could have speculated. WWI saw the introduction of the machine gun into modern warfare, for instance. How could Robbie be expected to function as the only person in his life who knows what they actually look like, what they sound like? Because of the time he lived in history, he was left without proper mental health care, and then when a situation escalated, he lost his mind and returned to battlefield tactics.
Star-crossed lovers
Robbie and Hannah belong to an archetypal pair that we commonly associate with Romeo and Juliet. They represent the young lovers who use marriage as an escape from the pains of life, but since they are "star-crossed," their passionate affairs are over in short time.
The allegory of secrets and the grave
The correct way to interpret the plot structure of the story is to say that Grace's role as the narrator makes her into a confessor, like a religious woman gaining her last rites before death. She is getting a chance to straighten her affairs, and she's also getting one last chance to do that one thing she's always failed to do; tell the truth about what happened. She cannot face the fear of death with a secret. She must say her peace.
David's death
David's death functions as a symbol in the novel for something, but it's hard to see what exactly. Part of the reason is seems confusing is because the war itself was confusing and sudden, so David's death would have occurred without explanation, and the truth is that in a bigger sense, David's death represents the deaths of many, many people. Therefore, the effect of David's death is that it casts a shadow over the duration of the story, a shadow that continues into the present day: the military industrial complex. Before WWI, wars were still being fought on horseback. After WWI, everything was permanently different, like David's death is random and terrifying and permanent to the girls.
