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Hello, you mentioned you have previously done this with 301 redirects. It is possible to do without penalties or losing rankings on your primary domain. However, the problem arises that you are essentially splitting page authority with the sub-domain which can potentially be treated as a separate website. I would not recommend using sub-domain for your forum to avoid the issue of competing with your primary domain. Forums can be a great source of content and traffic so I would give it the best opportunity to do so on your  domain. It would also be good to spend some time on maintenance to correct those errors and moderate poor quality / spammy / and thin content if necessary. If you do decide to use a sub-domain, you would want to make sure to provide relevant links on every page back to your primary domain. Best of luck with your forum.