by the words

"... for in the day that thou eatest thereof thou shalt surely die."

it didn't mean that they would die instantly. when they were in the garden of Eden, they were immortal, they were sinless because they knew not "good and evil" so they didn't need to die to "be purged"

when they ate of the fruit, they gained the knowledge of "good and evil" and that is the point that sin entered.

in Romans 6:23

For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord.

that is why they didn't die the instant they ate of the fruit. Death is a product of Sin

You have asked a very intriguing question, in order to try to derive what God meant when he said;

Genesis 2:16 and 17 KJV

16 And the LORD God commanded the man, saying, Of every tree of the garden thou mayest freely eat:

17 But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.

First of all note that in verse 16 God tells man that he may eat freely from every tree.

but in verse 17 he warns him that if he eats from the tree of the knowledge of good and evil in that day you shalt surely die.

First of all we need to know what God meant by the word die, and to ascertain that we must first know what life is; so lets take a look at man's creation.

Genesis 2:7 KJV

And the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul.

Please notice that it says that man became a living Soul, and that has to do with the eternal part of man and not the Physical man, and how do we know that ?

Genesis 6:17, 7:15 and 7:22

6:17 And, behold, I, even I, do bring a flood of waters upon the earth, to destroy all flesh, wherein is the breath of life, from under heaven; and every thing that is in the earth shall die.

7:15 And they went in unto Noah into the ark, two and two of all flesh, wherein is the breath of life.

7:22 All in whose nostrils was the breath of life, of all that was in the dry land, died.

Of all of the creatures God created only man became a living soul.

So when God told man that he should surely die he must have been referencing the soul, since he deemed man a living soul and not a living being.

That not only separates man from the animals, but also the Angels.

So let's take a longer look at Genesis 2:17.

God did not say in the day you eat of it you will die, instead he said you shall surely die. So what is the difference between the two?

The original Hebrew was:

מוּת

muwth (mooth) v.

1. (literally or figuratively) to die

2. (causatively) to kill

KJV: X at all, X crying, (be) dead (body, man, one), (put to, worthy of) death, destroy(-er), (cause to, be like to, must) die, kill, necro(-mancer), X must needs, slay, X surely, X very suddenly, X in (no) wise.

מ֥וֹת

mō•wṯ

surely

תָּמֽוּת׃

tā•mūṯ.

you will die

The difference here is in the words surely you will die and the word for kill which is:

נָכָה

nakah (naw-kaw') v.

1. to strike (lightly or severely, literally or figuratively)

KJV: beat, cast forth, clap, give (wounds), X go forward, X indeed, kill, make (slaughter), murderer, punish, slaughter, slay(-er, -ing), smite(-r, -ing), strike, be stricken, (give) stripes, X surely, wound.

notice that in the Hebrew it says surely you will die, not you will surely die.

God was not telling man that he would be killed but that his soul would die, which is quite different in that what God was saying was in effect that man would loose his immortality.

First consider this passage.

Hebrews 9:22

And almost all things are by the law purged with blood; and without shedding of blood is no remission.

Then go back to the fall of man.

Genesis 3:21

Unto Adam also and to his wife did the Lord God make coats of skins, and clothed them.

There was shedding of blood the same day of the fall of man.

Why did they, ultimately, not die?

They surely died ultimately, just that it didn't happen on the same day that they ate the fruit.

Altogether, Adam lived a total of 930 years, and then he died. (Genesis 5:5, NIV)

Why not on the same day?

That depends on how you translate it. NIV doesn't have any indication that they will die on the same day.

Genesis 2:16-17 (NIV)

And the Lord God commanded the man, “You are free to eat from any tree in the garden; but you must not eat from the tree of the knowledge of good and evil, for when you eat from it you will certainly die.”