Gen. 2:16-17, 3:21 – why didn’t Adam and Eve did die physically “on the very day” that they sinned (penal/capital punishment death)? An animal was slain “on that very day” on behalf of Adam and Eve to provide “garments of skin” for them (Gen. 3:21). But that was not the only thing the animal death did for them. God’s main concern was NOT about their physical needs, as any clothing from any material could have sufficed for that, but of the utmost concern was their spiritual wellbeing and forgiveness from the condemnation of their sin (substitutionary death). If that animal had not died as their substitute, then Adam and Eve would have been struck dead on the spot when God showed up “in the cool of the day” (as Ananias and Sapphira were). That was a penal death [capital punishment] – NOT a natural death (i.e. dying from a natural cause). That animal died Adam and Eve’s penal death for them, i.e., the death they would have died “on that day” if the animal had not died in their place. That animal did not die their natural death for them.
While the substitutionary death of the animal slain by God paid the penal death (capital punishment), and provisionally atoned for their sin, physical death would still be a natural consequence of being cut-off from the access to the tree of life à physical death would eventually and naturally follow (i.e. they were made from the dust, and now being cut-off from the Tree of Life, would eventually age and physically deteriorate until they return to the dust (Gen. 3:19)).
(NOTE: Jesus Christ did not die on the Cross to save us from natural death. He died to save us from our sin and its condemnation, which would have killed us by God’s decree instantly “on the day” we sinned unless He died in our place. He died our PENAL death for us – NOT our NATURAL death.)(Also see FEDERAL HEADSHIP for a deeper look into this and the effects/benefits on all mankind)
See also related “Topic Studies & Terms”:
“Death (Biblical Definitions)”
Related full “Study Series” (available upon request if not hyperlinked):
[For a more in-depth study see eschatology “Study Series 3 What Happened in the Garden”]