(I found that) - ~던데

던데 is used when you found that something was contrary to my previous opinion/expectation or opinions/expectations of others.

For example,

• 이미 하던데 = (I found that they) were already doing it

Replace 다 of verbs/adjectives with 던데

• 하던데 = (I found that he/she/they) was/were doing it
• 가던데 = (I found that he/she/they) was/were going (somewhere)
• 오던데 = (I found that he/she/they) was/were coming (here)
• 먹던데 = (I found that he/she/they) was/were eating
• 마시던데 = (I found that he/she/they) was/were drinking 
• 살던데 = (I found that he/she/they) was/were living (somewhere) 

• 많던데 =  (I found that there) were many
• 적던데 = (I found that they) were small (in quantity)
• 빠르던데 = (I found that he/she/they) was/were fast
• 느리던데 = (I found that he/she/they) was/were slow

Examples sentences

• 미술관에 사람들 적을 줄 알았는데, 사람들 많던데 = I thought/figured that there won't be many people in the art gallery, but there were many people.

• 짐 집에 있어? = Is Jim at home?
• 아니, 아까 어디 가던데 = No, a while ago (I saw him) going somewhere.

• 크리스마스라 시티에 사람들 많지? = There are many people in the city as it's Christmas, aren't there?
• 아니, 생각보다 사람들 안 많던데 = No, there weren't as many people as I thought.

You can also use 던데 when you saw someone/something somewhere doing something but now no longer quite sure what happened to them next, i.e. where they are or what they are doing now.

• 지니 어디 있어? = Where's Jinnie?
• 아까 샾에서 일하던데 = A while ago, (I saw her) working at the shop. (Though I'm not quite sure where she is or what she's doing now)

• 정수 어디 갔어요? = Has Jeong-su gone somewhere?
• 응, 친구 만나러 나가던데 = Yes, (I saw him) going out to meet his friend (I don't know about the details of where he is or what he's doing though.)