I'd be interested in a proper discussion about choosing between a subdomain and a folder, including the small details people tend to leave out. What would influence your decision for a new forum? Please describe the kind of website and the visitor's task. What would you change first, and how would you check that it helped on both a phone and a computer? A concrete example would be useful for someone maintaining a small site. If you disagree with a suggestion, explain what you would do instead and why.