First, we talked about the vowel "o." When you use the vowel /o/ from your native language, it is a short sound, and you can sound abrupt, rude or irritated when you are merely saying "no." To avoid this problem, lengthen your o and use the American diphthong /oʊ/. Want more help with this? Check out this video.
Next we talked about the pattern of stopping the "th" sound. This happens when you have your tongue behind your teeth and/or you don't allow the air to flow for a fricative "th" sound. The problem word we identified for this pattern is the word "third." If you stop the "th" in "third," it sounds like you are saying "turd." Find more help for TH on our playlist.
Lastly, we talked about using the correct word stress in phrases (stressing the last content word - noun, verb or adjective.). We used the example "I don't know," and pointed out that if you stress "don't" rather than "know," it sounds like you are correctly someone who said you do know. You can see more about this phrase in the video for that specific phrase, here.