To make the marinade, stir together the soy sauce, brown sugar, minced garlic, grated ginger, and water. Pour this mixture over your meat, tofu, or fish and marinate for 15-30 minutes, depending on the item.
To make the sauce, stir together the soy sauce, brown sugar, minced garlic, grated ginger, water, AND add an additional 1/2 cup water and 1 Tbsp cornstarch. (Do not turn used marinade into a sauce. Rather, make separate batches to prevent cross contamination.) Bring this mixture up to a simmer in a small sauce pot over medium heat, while whisking. Once it reaches a simmer, it will thicken into a shiny sauce. Pour or brush the sauce over your cooked meat, vegetables, or tofu.
To use as a stir fry sauce, stir together all of the ingredients in a bowl. Once your stir fry meat or vegetables are cooked, pour the sauce mixture directly into your hot stir fry pan instead of simmering separately in a sauce pot. The heat from the pan will simmer the sauce on contact and thicken the sauce (make sure the stir fry pan is very hot). Stir to coat your cooked vegetables or meat.