How can I effectively restore wood stain that has hardened in the can?

To effectively restore hardened wood stain in the can, you can try adding a small amount of mineral spirits or paint thinner to the stain and stirring it thoroughly. If the stain is still too thick, you can also try using a commercial wood stain rejuvenator or conditioner to soften it. Be sure to follow the manufacturer's instructions and test the stain on a small, inconspicuous area before applying it to the entire surface.