Quote 4
"[My legs] are full of trench-mortar fragments, old screws and bedsprings and things." (13.62)
Notice how smoothly Frederic brings things around from the battlefield of literal war, to the battlefield of the bedroom. We can pretty well imagine what he means by "old screws" but he makes it clear when he adds "bedsprings" to the mix.
Quote 5
"What’s the matter, darling?"
"I never felt like a whore before." (23.143)
This is probably the first time Catherine and Frederic have sex together outside of his hospital bed. Part of why she feels like a whore is because she knows that not just any hotel would let them get a room for a few hours, married or not. When they have sex in the hospital it seems like part of her nursing duties in a way, and therefore acceptable.
Quote 6
"I feel like a criminal. I’ve deserted from the army." (34.114)
We don’t know if Frederic ever gets over this feeling. We can see that if he hadn’t deserted, he would probably be dead, but Frederic isn’t sure it was worth it. He possibly even thinks Catherine’s death is punishment for his "cowardly" act. Interestingly, he has sympathy for other "deserters," like the man who is on the run and afraid to seek medical attention for his hernia because he thinks they will send him back to the battlefield.