I have a question... I want to run a forum about car modifications to allow members to register and post questions.. so I came up with a domain name, this is just an example; Exampleforum.ca after doing more research I noticed there is a forum already created at Exampleforum.com If I register Exampleforum.ca and offered the same experience as the already running Exampleforum.com site would that put me at risk of legal issues with Exampleforum.com they do not have any registered trademarks as far as I can see.
Did you try to google your question? I did a quick search and basically everyone is saying that if you have similar content (or as you put it experience) then yes you may run into issues. If your forum was about something totally different then you would've been fine.
I've literally done the exact same thing (registered a .ca forum name that was the same name and niche as a .com forum) and never had a problem in 10 years, but I suppose it would depend on how the .com owners feel about it. Some will come after you to protect their brand and others don't care if it's a small niche, as mine is. My advice would be to avoid it, just in case, or be prepared to lose the domain if it ends up in court.
So your business plan is to copy another website and expect them not to care? Apart from the futility of being a copycat where's your pride? And yeah, they can come after you but that will cost money and forums are less profitable than they were (but you know that from your business planning, right?) so they may opt to just let you have a go and give up.
Just put a variation like "theexample.com/forum/" or "forum.theexample.com" I think there is no need to put "forum" in the main domain name?
If you are asking the question, it is likely because you recognize, deep down, that it is an issue of concern. Why would you want to start a business under such a cloud? Start-ups are difficult enough without added, potential legal issues. Pick a name and domain that will not cause you problems in the future. Note that it is only after you have worked your butt off to make the site successful that your competitor will notice you and take legal action. That will always be looming over your business if you take this path.