here's the story
sometime in my class, the lesson is English. and at the speaking test, I used some slang words. not much, almost entirely still using formal form. but my teacher said that slang language is not suitable with English language rules. yeah, that's not formal language, and if we search in dictionary, there is no slang word. but, "not suitable with English language rules"?
BUT, the fact is, if we speak use "textbook language" in other country like US etc, they will feel queer with it. and, it's awkward.

then the problem is : What's the function of learning English? IF we CAN'T and DON'T want understand what are they talking about? what do you think?

CMIIW . sorry if my English is horrible