What a question!
English is their mother-tongue! Maybe they are not so good at grammar (of course this 'good' defines in a Chinese way, it doesnt mean that is useful。
Its a sad story about Chinese learning system), but if they wanna make a Chinese not to understand what they talk about, I reckon its just a piece of cake。
Just remember one thing, if a Chinese person who has a 8 or 10 (whatever) degree in English, maybe he or she still cant speak much in English。
So as a Chinese, you should start to think about why you learn another language? what is it for? and how to learn it? You should not just take whatever the teachers or the Chinese system tell you to do。
。