Genesis 2:16-17

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

Thou shall die — That is, thou shalt lose all the happiness thou hast either in possession or prospect; and thou shalt become liable to death, and all the miseries that preface and attend it. This was threatened as the immediate consequence of sin.

In the day thou eatest, thou shalt die — Not only thou shalt become mortal, but spiritual death and the forerunners of temporal death shall immediately seize thee.