-더라, -던, -었던, -던데, -더니, -었더니
These verb endings I think are among the hardest things for foreigners (but I can only speak for myself) to wrap their minds around when learning Korean. I don't really get them yet. But until you learn how to use them correctly it seems like your Korean will never sound fully natural.
If at some point you had a lesson to clear up exactly when and how to use these, I think it would be a big help for your intermediate to advanced learners.