I like to see 던데 as serving two purposes. 1) to soften what you’re saying, and 2) tell the listener what you’re saying is based on your recollection or experience. The two kind of go hand in hand. Lets look at a couple of examples.
Example 1: Consider the two sentences
- 나 와인 싫은데
- 나 와인 섫던데
The first sentence is factually stating that you don’t like wine, and ending the sentence with the usual nuances of 은/는데. The second sentence is a softer way of saying that you don’t like wine based on your recollections of wine in the past. In this way you still communicate that you don’t like wine, but it’s softer because its less factual and more subjective based on your personal experience.
Example 2: Consider the situation where you’re out shopping with your wife, husband, or whoever and you think you need to buy some toilet paper
A: 우리 휴지 사야 될 것 같아
B: 집에 있는 것 같아
A: 집에 없던데
If you say 없었는데 it is factually saying that there isn’t any, but if you say 없던데 it implies that you had a look recently and saw that there wasn’t any.
Example 3: Consider the situation where you’re explaining how to do something to someone else
- 이렇게 해야 하던데요
By drawing on your past recollection and experience you are softening the instruction to the listener because you’re not stating factually that this is how you have to do it, but its how its done based on what you’ve seen in the past. It’s like saying “you have to do it like this” versus “from what I’ve seen you have to do it like this”.
Korean grammar tends to have many ways of softening what you say. 은/는데 is actually one of those ways. In example three if you say 이렇게 해야 돼요 it would even be more direct. 던데 is like another level of softening what you say.
If you found this explanation helpful you may want to read more about the 더 grammar particle. Or if you didn’t find this helpful maybe reading about the 더 grammar particle as an introduction will help.