I had this as a "type what you hear" task, so I typed what I heard: Vi sjunger. Wrong. Three questions later it came up again. This time of course I typed, "De sjunger." Wrong again! After the timed practice session was over, I used the "scoreboard" to review the lesson (I don't know if everybody has it, I think I am in a test group). I was able to play the two recordings in juxtaposition, one after the other. They sound EXACTLY the same! :(
Thanks Arnauti. I guess this confirms what linguists say about language acquisition... that while adults do just fine in learning vocab and grammar, there's something unique about the ability to hear and imitate individual phonemes during childhood. I really couldn't hear any difference.
From watching normal Japanese postdoc students learn English, a normal middle-aged American professor learn German, and having some experience with neuronal networks: it is clearly a problem to hear sounds that don't exist in your mothertongue in the beginning - but it is doable at any age, and the best method is to find a description how the two sounds you have trouble distinguishing are formed in your mouth, and then listen and try to speak after a native speaker or two good recordings of those two sounds. For days. Everywhere you can. R/L; R/L; R/L... U/Ü, U/Ü, U/Ü.... In addition, it is good (or maybe even completely necessary) to have somebody who can give you feedback whether you are pronouncing them correctly, especially in the beginning.
The reason it is so hard to hear the difference is in your brain: our brain is trying to recognise a familiar pattern (think faces in the wall or Jesus in a biscuit...) so you always "hear" (recognise) the sound you are used to. So what you need to teach it, is that the difference in the neuronal firing pattern is actually important, and, roughly speaking, what the tolerance for each of the two groups is.