OK. I fixed the forwarding. What my hosting said was I had to firstly change to their servers (after another help tech told me to set an A Address Record to my own hosting IP Address at my registrar), and then had to re-create the forum sub-domain, then had to use their cPanel to redirect the forum to the new place. No .htaccess redirect appeared to work. It also means I can't do a blanket sub-domain redirect to my /forum/, which would have been kinda nice catch-all, since I'm not sure what other sub-domains the previous owner had.
Those folders were in the .htaccess file of the domain which was being redirected to. not in the .htacess file of the domain under discussion. which was why I asked if I had to change the names of the folders to those of the problem domain. Did that make sense?