What do you think about babies and how they learn to speak? Do they know any grammar rule? Do they know any phonetics hint? Surely, they don't. So why do children master such difficult languages and Arabic, Japanese or Russian with no effort? What have adults forgotten that made them able to master their own language? Thus, because of the fact they've forgotten it they can hardly learn at least one more language.

The answer is as simple as you have never thought of. They just do not bother. That's true, a child doesn't care if it pronounces sounds properly or not, or if it uses the right tense or wrong one. The child just has its aim - it should make everything possible to make those big guys understand it and do what it wants. And it doesn't matter if it says