How to conditionally show/hide form inputs?

I am using a form with many input fields (1; 2;3;4 & so on)
I can make fields 2, 3 & 4 remain invisible and become visible when the previous one is no longer empty.
But on entering row 2 I cannot make row 1 disappear.
When I try then the above solution in reverse, (with regard to making items visible) then it too stops working.