If brown sugar hardens:

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.

To soften brown sugar quickly, place open bag of brown sugar in microwave oven. Set 1 cup of water next to the bag in the microwave. Microwave on High (100%) 2 to 3 minutes, checking often, until brown sugar is softened.