Your English is very good. The only errors I see are : 

"When I get tired I sometimes watch exactly same dreams that I somehow teach a bunch of Predators (from a very well-known movie) international politics which is my major. "

I would write : " When I get tired, sometimes I have the same dreams where I am teaching a bunch of predators (from a very well-known movie) about international politics, which is my major".

Another error I see is : " Since it always seems like they are keenly eager to learn about China, I teach them when Qin Dynasty has finished, why the Chinese government has put censorship on its people, how the China uses military forces to take over Spratly islands and so on…. "

I would write : " Since they always seem to be keen and eager to learn about China, I teach them when the Qin Dynasty has finished, why the Chinese government has put censorship on its people, how the China uses military forces to take over Spratly islands and so on…. "

All in all, only minor errors.