In an introductory note to this chapter, the Kitzur Shulchan Aruch writes that most Jews are already familiar with the prohibited labors of Shabbos and that he is only addressing common situations with which people may not be familiar. Clearly, the laws of Shabbos constitute an area requiring much greater study than this email alone will allow.
80:3 It is prohibited to pour a boiling liquid onto pieces of bread or matzah. Rather, one may pour the liquid into a dish, let it cool off a bit until it is fit to be eaten, and then place the bread or matzah into the liquid. (The temperature it must cool past is what we call “yad soledes bo,” so hot that one jerks his hand away from it – Mishnah Brurah 318:45. This is around 110 degrees.) However, so long as the liquid is boiling, even if it’s in a bowl, one may not put the bread or matzah in it. Similarly, one may not put salt or spices in a liquid as long as it is boiling, even in a bowl and certainly not in the pot. (Mishnah Brurah 318:65 permits it in a second vessel even if the liquid is still yad soledes bo.) What one should do is wait until the liquid cools enough so that it is fit to be eaten. Some authorities are lenient when it comes to salt that was boiled as part of its manufacturing, but the Kitzur Shulchan Aruch blesses those who act stringently in this matter. Similarly, boiling coffee and tea should not be poured into a cup to drink from that already has sugar in it. He should first pour the tea or coffee, and then add the sugar. In a case of need, one may act leniently in this matter.
80:4 We may not place fruit or water on a oven after it has been lit because it’s possible that the water will boil or the fruit will cook. Even if one only wants to warm these things, if it’s possible that they might get cooked, one may not put them there to warm. Similarly, something dry like a piece of kugel that has some fat on it should not be placed by a fireplace or on an oven where it might come to cook even if one’s intention is just to warm it. If it’s impossible for the item to cook, and it can only be warmed, then one may put them there, even if the fat is congealed or the water has frozen from the cold. (The practice is to act stringently in this matter but in a case of need, one may act leniently – Rema 318:16.) However, one may not warm cold food in an oven even if it would be impossible for it to cook there. If doing so is necessary for a person who is sick, though not seriously so, an authority should be consulted. There are some who place items back in the oven on Shabbos to keep them warm based on the fact that they’re still hot. (This leniency only applies to items that actually came out of the oven – Shaar HaTziyon 253:58.) If these items had cooled off completely, this is forbidden; the Kitzur Shulchan Aruch recommends that a conscientious person refrain from doing so in all circumstances.