A warning about the consequence of an action is sometimes indicated with the word פן (lest) (ולא תגעו בו פן תמותון - neither shall you touch it, lest you die⁷⁶⁹) and sometimes with the word לא – (ושמרתם..ולא יהיה קצת). And after investigation, one finds that the word פן is usually correct with a warning not to do something, while the word לא comes with a warning that one is obligated to do something. And if the consequence of the action obligates on its own – it will use the word לא. And if it does not obligate on its own, it will use the word פן.

An example of the consequence of an action obligating on its own is if one is about to die and he is given a way to avoid death by performing a certain action. Then it will use the wordלא.

And an example of the consequence of an action not obligating on its own is if one does not do as he is commanded, he will die. Then it will use the wordפן⁷⁷⁰.

footnotes: ⁷⁶⁹ Genesis 3:3. ⁷⁷⁰ Malbim, HaTorah v’HaMitzvah Shemini 31.