I found a cake recipe that wants maple sugar in it, but I don't have maple sugar. Can regular sugar be substituted?
If maple is intended to be a significant flavor in the cake (i.e. you're making a maple cake, or a maple rum cake) it's not going to be the same without it. The best substitutions would be maple syrup, or white sugar plus maple extract, so that you still get that wonderful flavor.
As for quantities, looking around online, the common recommendations for replacing 1/2 cup of maple sugar seem to be:
If you can do without the maple flavor, then you could just use the 3/4 cup white sugar, but I suspect it wasn't a superfluous ingredient!
In the case of this recipe, it looks like the maple is important, but there aren't liquids to reduce. That probably means that the easiest option is the maple extract, but you could also take the recommended amount of maple syrup, and boil it down until you've lost the right amount of water. (For example, for 1 cup of maple sugar, take 2 cups maple syrup and reduce it to 1.5 cups.)