Question

that place is in the south of the city其中介词用in 为什么,在什么情况下用on,

Answer 1

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 not in China,but on the south of China.

To say "at the south of..." is not common.It appears in some phrases such as:

He arrived at the South of Human.