During a cooking lesson last week we needed some brown sugar. My student had plenty—five bags—but all were about half-full with rock-hard brown sugar. We chiseled with a knife to get a half cup, but there is an easier way.
The hardening occurs when the brown sugar loses its natural moisture. To prevent this, place a small piece of bread in the container or bag of brown sugar. Hardened sugar will regain its softness in a day or two and newly-bought sugar will remain soft.