Hi. This is Jean. I just read your post, and I just wanna share something. I've been dealing with Koreans for 5 years now. The first thing that I noticed about them was that they are not so familiar with the English sounds. The sounds of Korean are totally different. So, you can start with giving them some of the critical sounds in both the English and the Korean Languages. Please give them comparative sounds analysis.
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Thanks!
Have a great day!