Most white sugar comes in simple paper packaging, which fails to protect it from moisture. Exposure to moisture causes hard lumps to form in the sugar. While lumps don't decrease food quality, and sugar will keep indefinitely in paper packaging, your recipe probably requires smooth, granulated sugar free of lumps. Once you have fixed your lumpy white sugar, store it properly to avoid annoying lumps in the future.
Pour sugar through a sifter to separate out the lumps. Use the sifted, lump-free sugar as usual.
Place white sugar lumps into a food processor.
Process the lumps on medium-high until sugar is granulated again. The sugar may be coarser than usual but will work fine in recipes.
Store remaining white sugar in an airtight container, such as a canning jar, and keep it in a cool and dry location.
Do not store sugar in the refrigerator or around other strong-smelling foods, as sugar absorbs odours.