던데

I saw that..., and?

Reports something you witnessed and leaves the door open for a reply: what they call it, that maths looks a bit hard for him, that this doesn't seem to exist in other countries. The 더 part means you saw or felt it yourself; the 는데 part means you are not finished, so it invites a response or sets up a contrast. Compare 더라, which reports the same first-hand observation but closes the sentence. All these lines are plain speech; 던데요 is the polite version.

Register: casual, plain speech