I tried to find something on Google but I couldn't, so I want to know the rule for using the particle の at the end of a sentence... when should I add な to の and say "なの" and when I should only say "の"? Thank you.

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I think that's one of the toughest questions for native Japanese speakers. I would put なの at the end of the words that end with う, like "そうなの(sou-nano)" or "どうなの(dou-nano)". and I would put の the words that end with く, る, む.
"行くの" iku-no
"するの" suru-no
"食べるの" taberu-no
"読むの" yomu-no