It is easy to say and write to enforce your "English only rule" but the class becomes a battlefield rather than an educational setting. Think of a time when you had to learn by force. Not because you wanted to but because you had to. Did you really retain that information or just did what you needed to in order to get pass that time period or that day? Now think of a time when you learned something because it was interesting and you wanted to learn it. I bet you still remember it up to now. The key is not in ruling the class with an iron fist, but rather making the class with an atmosphere where the student's interests are peaked, and at the same time having ground rules that are realistic and achievable. Telling your class to speak "English only" when the class is not equipped to do so only sets them up for failure and frustration on both sides. Once they break the rule, confidence goes down in their learning abilities, and you've loss that student. Of course, you'll have success with students who are average to above average in English, but that is not the case with most ESL students, specially not in Korea. But as educators, our duty is to help every single student and not just the advance few. It is easier to rule and force the class into submission with iron fisted rules like "English only" than to come up and actually think of ways that might improve learning for everyone. It's not that difficult, it just requires a little bit of research on the educators part and all the information in this day in age is on the web. So definitely have ground rules, but make them rules that students can achieve and research methods of teaching that are effective, that will peak students interest, that will make learning fun, and that will create an atmosphere that is beneficial for everyone.