Brown sugar becomes hard when moisture in the sugar evaporates. Therefore, the various methods used for softening brown sugar are intended to return moisture to the sugar.
To soften hard brown sugar, place an open bag of sugar in the microwave with a cup of water next to it. Microwave on high (100%) for 2-3 minutes. If your microwave doesn't have a carousel, turn the bag after each minute.
NOTE: This works great, but isn't a permanent fix. Any unused sugar will dry up again. However, the process can be repeated each time you need sugar.
Place about 1/2 lb. of hardened brown sugar in microwave safe bowl. Cover sugar with two pieces of wet paper towels. Tightly cover bowl with plastic wrap. Heat in microwave at HIGH for 1 1/2
to 2 minutes. Divide sugar with fork (sugar will be hot); stir. Use immediately.
NOTE: Microwave ovens vary in power; cooking time may need adjustment.
Place a piece of foil or plastic wrap directly on the sugar. Set a piece of crumpled, dampened paper towel on the foil. Cover container tightly. The sugar will absorb the moisture from the paper towel and become soft. Remove the paper towel when it has dried out.
Place about 1/2 pound of hardened brown sugar in a bowl. Cover sugar with two pieces of wet paper towels. Cover bowl tightly with aluminum foil or plastic wrap. Let stand overnight at room temperature. Divide sugar with fork; stir
and use immediately.