[OT] Thou shalt surely die

"In the day you eat of it you shall die" (Gen 2:16-17). 

The serpent challenges the truthfulness of this statement, and Adam and Eve die much later.  Common explanations for God's seemingly inconstant statement are:

• Day may mean time
• Cruel conditions out of the garden may be like death
• You shall not attain the measure of life originally allotted to you (1000 years)
• You deserve death, which will be fulfilled in due time
• This is an exaggerated statement to motivate Adam from further sinning
• Adam was to die until he repented
• You are now immortal, but shall become mortal -- a type of death
• Separation from God in the garden comprises a type of spiritual death
  

Cassuto's explanation is that Adam will not be allowed to eat from the tree of life, which would have prevented him from ever dieing.  After choosing the fruit of knowledge, he was not allowed to eat of the tree of life, ensuring he would eventually die.