Understanding the type of knots that are allowed to be opened on Shabbos.
הכנה ליום אחר (Preparations for a Weekday): Washing Dishes on Shabbos for Sunday. Releasing a Halachic Vow (הַתָּרַת נְדָרִים);
Moving a Candelabra That Has Oil in It on Shabbos. Using a Dimmer on a Light on Shabbos;