This is not an easy question to answer.Let's discuss some examples:Guangzhou is in the south of China.--- Duangzhou is in China,but in the south of China.India is on the south of China.--- India is no...India is on the south of China，可是老师说在外部的话是用TO，那to和on 到底该怎么用to 也可以, 有接临,接壤的意思。For example, Hunan is to the south of Hubei. That means, not only is Hunan on the south of Hubei, they also share a section of boundary. Usually, we do not say "Hunan is to the south of Henan". However, we could say Hunan is on the south of Henan.