Sugar that has been hardened?!

I agree with Tom about the brown sugar, though the apple slices are my preferred way to go, the sugar stays soft.

With white sugar, the back of a spoon will usually work, but if you have a lot, I'd throw it in the food processor or blender and pulse-don't turn it into confectioners sugar. ;)

White sugar may clump, but it can be broken up with your fingers

Brown sugar is trickier. To soften hardened brown sugar, you can try a couple of methods:

Place a slice of apple or a wet paper towel the container with your brown sugar and seal it. In a few hours, your brown sugar should be soft again. If that isn't long enough, let it sit sealed overnight.

OR

Put it in the microwave for a few minutes on low. Only microwave as much as you need, and use the sugar immediately. It will become hard again when it cools.

im guessing its brown sugar. it clumps together because it has water and a spoon of molassus in every cup.

if its white sugar, id try mashing it up first, it could be clumped because of condensation or if its a bit "damp" suring the summer.

just try softening it with your fingers, if its super hard and wont come appart (or somes appart in unbreakble clumps) i sugest buying some more.

Moisten a paper towel and put on top of sugar in a microwave safe container. Heat in increments of 6 min keeping a close eye on it, then stir.

If you just pound it and it breaks then it should be fine.
But if it doesn't I would suggest getting new sugar.
:)