Bread Question

O.K. I finally got brave enough to play with a recipe, change some of the ingredients and bake it in the bread machine. It turned out so well I thought, well, is there a basic law of substitutions. Page, I know you do this a lot right?

Is flour interchangeable? If it calls for 3 coups of bread flour can I use 3 cups of wheat flour? Or 1-1/2 cups each? Or 1 cup each Bread, wheat and soy?

What about liquids? I followed some advice given to me here and replaced all the liquid in a recipe calling for buttermilk and water, with milk and it was fine.

Do you get the gist of what I am asking? I would like to experiment with the sweetness of loaves too, what difference does sugar, honey, karo, or maple syrup make?

what about adding hard ingredients like nuts, seeds and oats? Do I add it all up to make a certain amount or does it matter?

THANKS!!!