Learn the גמרא דף לח: “א”ר זריקא (seven lines from the top of the Amud) until the Mishna
Questions:
What is the reason why if the pot is placed on the ground it would be prohibited to return the food to the stove?
See רש”י “אבל הניח על גבי קרקע
What is the uncertainty in the case of פינה ממיחם למיחם (transferring the food to a different pot) whether it is permitted to return the second pot to the stove?
See רש”י “פינה
Learn תוס’ “פינה
Learn ר”ן דף יז: דפי הריף “בעי רב אשי
https://hebrewbooks.org/pdfpager.aspx?req=37953&st=&pgnum=584&hilite=