I guess it depends on what you commentate on. Magic words are "In My Opinion," also "I Don't Agree." Things like that. Make it clear it's just your opinion and in no way fact. Like if you're watching a Basketball game and you say "I don't agree with LeBron's play here, in my opinion the wiser decision would have been a cross over into a fade-away jumper." I think certain footage you can get away with under the copyright law if you're commentating or doing a parody. Don't quote me though, I haven't researched that part of things.