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.
80:7 If one did not add vinegar to condiments like mustard or horseradish before Shabbos started, he may not do so on Shabbos unless he does so in an unusual fashion, such as by first pouring the vinegar into a dish and then adding the mustard or the horseradish, instead of the other way around. One may not mix the ingredients into a thick paste; there must be a large enough quantity of vinegar to ensure that the mixture is thin. It should not be mixed with a spoon or similar utensil, but rather stirred with one's finger or shaken in a bowl until it is mixed. (If one added a little vinegar before Shabbos, he may increase it on Shabbos. A few drops before Shabbos, however, is insignificant – Mishnah Brurah 321:65.)
80:8 Fruit found under a tree may not even be moved because it may have fallen on Shabbos. Similarly, fruit in the possession of a non-Jew may not even be moved if there is a possibility that it may have been picked on Shabbos.