What does Genesis 3:19 mean?
This verse concludes God's curse on Adam for his sin, and it has a devastating ring of finality. In the previous two verses, God revealed that Adam's working life in the fields would be marked by pain and frustration. The ground itself would be cursed, making it difficult to get the crops they would need to live.Now God concludes by saying that instead of work being a joyful source of purpose and meaning in Adam's life, it would be a lifelong source of necessary frustration. It would be hard and sweaty. And it would end in Adam's eventual death. God, who formed Adam out of the dust of the ground, announces that Adam will one day die and return to dust. Death would be the final consequence of Adam's choice to sin, just as God had warned when giving the command.
It's true that Adam did not stop breathing on the day he ate of the tree, but death entered into his life on that day. In modern language, we sometimes refer metaphorically to a person with a fatal injury or disease as "already dead." Adam's heart may have continued to beat for many years, but the poison which killed him entered his body when he sinned.
In addition to being separated from God's presence in a spiritual death, every day of Adam's life from this moment on would be marked by an awareness that he would one day die. That's the curse all humans have lived under ever since. For those in Christ, though, the curse of death will be overcome (Ephesians 2:1-10).