1. La deuxième phrase 'il ne fait pas de natation' sonne même mieux! So for me the second is even better than the first one! So it's correct :) 2. Ils sont revenus de Corée hier matin. Yes correct 3. Mes* (plural) parents mangent chez des/tes amis. It depends of who you talk to. Example : you speak to a friend and your parents eat with his own friends, you will say 'mes parents mangent chez tes amis' but if you speak to anyone 'chez des amis'. I know the first it's pretty rare but it's possible, and if you say 'tes amis' it's not you're friends so can be weird. Finally, 'des amis' is better but the second is totally possible, just it can be negative.