It also has seemed to me that in learning a language, there are four relatively independent skills that one must master: reading, writing, listening, and speaking. Each one is pretty much acquired on its own, though obviously there is overlap. But one can be good at reading and weak at writing, and similarly with any other pair: good in one, weak (at first, until the skill is acquired through practice) in the other.

Great article. My daughter teaches Latin and Greek at a Friends school. I sent her a link to the article.