Ebrahim Bawazir
In simple words, both substance needs liquid to work but, Baking Soda (sodium bicarbonate) needs an acidic substance to work as starter to be able to activate and begin the rising process, wich noticed in many cases by adding lime juice or some sort of acid in the recipe, on second hand Baking Powder is a mix of dry (sodium bicarbonate) as well as dry acid, so it only need a liquid to be able to activate!
in terms of substitution as @baneeks said you would do 4tsp of baking powder in substitute of 1tsp of baking soda (with the elimination of the acid in the recipe), and if you would do the opposite you need 1/4tsp of baking soda and an acid (the rule might be wide depending on how strong the acid you use but you can do 1tsp of lemon juice or vinegar for every 1/2tsp of baking soda) instead of 1tsp of baking powder!