Calling
Hide makes the form "disssapear" but it's still there, and since it is shown modally the focus can't return to the caller.
As Circular says, you have to close the modal form either calling
Close or, which is better, setting its
ModalResult which incidentally also closes it.
By the way, you should check the result of
ShowModal to allow for the user closing the form without doing anything or clicking a "cancel" button or similar:
if (ZubAll.ShowModal = mrOK) {or mrYes or whatever}
and (ZubAll.Sprung.Caption = 'ABB') Then Begin
calculated := False;
Break;
End;