* -던데 => relates what one saw or experienced. (던 indicates the past) * -는데 => states a fact in connection with some other thing. (는 indicates the present) * 어제 보니까 자전거를 잘 타던데 언제 배운 거예요? => 잘 타는데 would mean more or less the same thing, but it doesn't work as well with 어제 보니까 in the past tense. A 소영 씨가 기침을 계속 하던데 ... = I saw 소영 continue to cough, ... B 소영 씨가 기침을 계속 하는데 ... = 소영 continues to cough, ... (A) specifically says that the speaker saw her coughing, while (B) makes a general factual statement. When both can be used, -던데 usually makes it clearer and has a greater impact.

this is what I know... 던데 is used when you're recounting (and recollecting) a past event, that recollection is your background information