But of the tree of the knowledge of good and evil: but translates the common Hebrew linking term, which here leads to an exception to the permission to eat given in verse 16. For the full expression see verse 9.

You shall not eat: this may also be rendered “you must not….” Some languages may make use of the idea of taboo here; for example, “There is only one tree which it is taboo for you to eat its fruit….”

For in the day that you eat of it is literally “for on the day [of] your eating from it”; that is, “the very day you eat from it,” or “straight away.”

You shall die is the same Hebrew double verb construction as is translated “freely eat” in verse 16, and here the meaning is “you will certainly die,” “you will die for sure.” The emphatic statement may also be translated “you will die on the day you eat it.”

Quoted with permission from Reyburn, William D. and Fry, Euan McG. A Handbook on Genesis. (UBS Helps for Translators). New York: UBS, 1997. For this and other handbooks for translators see here .