He became subject to physical suffering, sickness and death. He lost his innocence, became unrighteous and unholy, guilty and lost, an enemy and an alien. If he died in his sin, he would suffer eternal doom. His sinful nature was passed on to all his posterity. Adam could only beget children with his own nature, and that nature was sinful.
All children have to be taught to do right, but they know how to do wrong without being taught.
As an illustration of this principle, a metal jelly-mold gives its shape to all the gelatin desserts or salads that are made in it. If you should drop the metal pattern and it becomes dented, all the future gelatin molds will show the effects of the fall. Adam acted as a representative of the human race. Since we are all created as free, moral agents, perhaps we would all eventually have done the same as Adam did anyway. It depends on whether you are looking at it from God's standpoint or man's.