> The only thing that seems to be different between adult and child learners is acquiring specific sounds/tones.
It isn't actually different. It appears to be different, because people conceptualize the problem backwards, as learning to distinguish two sounds that, in the beginning, sound the same.
But what actually happens is that babies are born distinguishing all linguistically relevant sounds, and learn not to distinguish the sounds that their language considers equivalent. This ability is retained by adults.
That can be very difficult. Fundamentally, you need to keep trying to tell samples of the two sounds apart until, eventually, you figure it out. You will need a trustworthy source for the sounds.
It will probably help if you practice producing the sounds too, but that's not enough.
A friend of mine put in a lot of effort to learn English by listening to the radio. And her English is very good.
But like most Mandarin speakers, she can't tell the difference between "th" (as in "thick") and "s" ("sick"). I was able to teach her how to produce "th"; that was easy.
Since she learned by listening instead of reading (which is the correct way to do it if you want to interact with people rather than books), she has no mental model of which "s" sounds in English are real "s" sounds and which ones are secretly "th". So if you talk to her now, it will be essentially random whether any of those sounds is produced correctly or as its evil mirror version. You'll hear a lot of stuff like "thingle".
It's not obvious to me that this is an improvement over her original practice of using "s" in all cases.
It isn't actually different. It appears to be different, because people conceptualize the problem backwards, as learning to distinguish two sounds that, in the beginning, sound the same.
But what actually happens is that babies are born distinguishing all linguistically relevant sounds, and learn not to distinguish the sounds that their language considers equivalent. This ability is retained by adults.