Permissible ways to heat up food on Shabbat

1. According to Ashkenazim, a pot of food, which was taken from the fire, one may not be returned to the fire unless it fulfills the following conditions. 1)the fire is covered (with something that upon which one wouldn't cook during the week like a blech), 2)food is fully cooked, 3)the food has not completely cooled off, 4)the pot was taken with intent to be put back, and 5)the pot is being held (and may be placed on a dry area except the floor while being held).
2. If one lacks either of the last two requirements (intent, being held), one may return a pot of food to the fire even as long as the food is needed and the other four requirements are satisfied.
3. If one lacks both of the last two requirements (intent, being held), one may return the pot to the fire only if one will not have hot food on Shabbat if one doesn't return it to the fire.