unsure how to wite IF formula

This question is similar to my last one. How do I write the formuanother IF formulala for d21 to add all cells from d5 thru d20, but not show a zero in d21 prior to entering values to be added. My last question was for a product formula and this board gave me an "if" formula. I'm guessing this is basically the same idea, but I don't know how to form it.