> 1. Accents are evidence that the speaker did not fully acquire the phonology of the language. A French native speaker will speak English with a French accent because they haven't acquired the full English phonological inventory. Children who learn a language don't have this problem.

True. But, referring to my earlier comment[0], this is a matter of changing language learning habits for adults, not a matter of ability.

> 2. Children will learn colors and shapes just fine without explicit instruction. They do it all the time.

Define "without explicit instruction". We explicitly read our children books that put a ton of emphasis on getting the words for colors, shapes and animals across (by repeating them over and over). We repeatedly ask our children questions such as "What color is this?", "What's that animal called?". If that is not teaching, I don't know what is.

> In the pre-developed world, children didn't get taught how to speak. There were no flash cards or toys for learning numbers. They just learned by observing.

Well, they certainly must have been listening, too, and their relatives must have pronounced the words for colors in the first place or otherwise they certainly would have never learned those words. But where do you draw the line here between "passive observing/listening" and "being taught"? To me, at the end of the day, these are all just different learning techniques and I don't see anything special in the way a child's brain acquires a language compared to an adult's.

> No one will passively acquire how to write.

Funny, I actually disagree here. I remember that, back in the day, my 4-year old cousin used to copy books verbatim, letter by letter, before he could actually read or write. (Where I'm using "write" in the sense that one puts letters on paper to form words and sentences and to articulate some meaning.) By the end of this whole process, my cousin knew how to read and write perfectly. And by that I mean: He was an absolute grammar nazi by the age of 6. Later, he would then go on to read dictionaries in foreign languages aloud, page by page, for hours while we were on vacation – just to annoy me. Today he speaks four languages absolutely fluently and he's at least somewhat proficient in another two.

Anyway, I'm sure my cousin once had to ask my aunt about the pronunciation of individual letters of the alphabet – to map them to the sounds he already knew – and probably also about words whose spelling differed a lot from their pronunciation. But if what he did is not "passively [acquiring] how to write" (and read), I don't know what is.

Then again, he put in massive amounts of time to accomplish what he might have learned much faster in elementary school.

> 4. Children make errors in production all the time. As they learn a language they make generalizations -- generalizations that actually make sense, like "mines" -- but which are considered "wrong" by adult speakers. They'll correct themselves over time without instruction.

Sure, as will adults. It's just that correcting them (whether a child or an adult) leads to much faster results and stops mistakes from becoming ingrained in their mind. Once again, it's a technique speeding up the learning process. (Which is why it's so helpful to have a native speaker correct you while you're learning a language.)

> You might want to check out reading Steven Pinker's "The Language Instinct," it has a lot of ideas and research that might be new to you.

Would you mind summarizing some of these ideas that, if I understand you correctly, undermine OP's (and – by extension – my own) argument?

[0]: https://news.ycombinator.com/item?id=27917214