(A-1)(C-1)(D-1)

For the life of me, I can't remember how to do this! I think I'm good with the first two by using the FOIL method, but I blank out with what to do with (D-1).

(AC-A-C+1)(D-1) = ??

I know it's easy, but I'm having no luck with my searches for the way to solve this problem.

Any help is GREATLY appreciated!