This is just a brief note...Later there will be lots more. The color of sugar is determined by its level of refinement--eradication of molasses. The darker, the more likely it is to clump and get hard. Do not discard. Place in the microwave and heat in 45 second increments. It will soften. don't over do it or you will have a liquid. Alternatively a large amount can be covered with a cvloth and hit hard with a heavy pot to break into chunks. Powder in food processor. A more old-fashioned way is to put the sugar in a mortar and grind with a pestle. This still may be necessary for the last little bits.
It will slightly improve the prevention if the sugar is put in an air-tight container along with a dry chunk of bread to absorb any moisture.