In Soldier's home, what is he teaching us about how people act in the world?
I don't think that Hemingway's intention in writing "Soldier's Home" was necessary to teach about how people act in the world. Rather, the story's narrow focus is the effects of combat on the human mind and spirit. Krebs, a World War I veteran, cannot adjust to normal life with his family after his experiences overseas. His parents and sister want him to be the same innocent young man who left college to go to war; this is impossible for him. He suffers from shellshock and wants to be able to feel emotions, but he simply cannot do so.
Hemingway uses the story to show that trauma such as being in combat does irreparable damage to many who experience it and that it is impossible for others who have not experienced the same thing to truly understand what victims of PTSD (shellshock/Post Traumatic Stress Disorder) endure.
Alternatively, perhaps you are referring to the way people place the expectations from the past on a soldier who has suffered incredible changes as a result of the way while people continued with their lives as usual. Once the soldier is home, he realizes the massive disconnect that occurs between people who have gone through life changing events and those who have continued business as usual.
It is sad that the soldier had been on the verge of death in several occasions, appreciating whatever he can from life while others take it for granted. Then, he returns a changed man and the others resent the fact that he has changed, refuse to listen to his needs, or refuse to understand him. This is perhaps what little message is being sent about the world outside the soldier's mind in the story.