I have been working on Vietnamese for several months now, and my pronunciation is at a brick wall. It has to do with the tonal structure. Before you assume the obvious, let me say that I am no stranger to tonal languages. I speak Chinese quite well, and Thai passably. But Vietnamese is different, somehow. Internalizing the tones seems to be a great deal more difficult. I've come up with a several possible reasons, none of which seem totally satisfactory: 1. Just as some languages are stress-timed while others are syllable timed, so some tonal languages are "stress-toned", i.e., like Chinese, where some syllables modify or lose their tone depending on the environment, while Vietnamese is "syllable-toned" - each syllable must have its tone clearly articulated; 2. Vietnamese tones contrast with each other more than in Chinese or Thai; 3. maybe Vietnamese speakers are simply less able or willing to tolerate tonal mistakes by foreigners than Chinese or Thai are; 4. maybe I'm just getting too old to learn another language well. The fact that the number of tones used in Vietnamese varies depending on the dialect (five in HCMC, six in Hanoi, and a confusing mishmash in between - I'm in Nha Trang, and my teacher insists that there are six tones, although the only difference between two of them is vowel length - I can't hear the difference!) doesn't help matters. In fact, the idea that there is a standard "Vietnamese" is debatable; even TV programs vary considerably, with the news usually in Hanoi dialect and the entertainment in HCMC dialect. Does anybody know of any research on this topic? I can't find anything.