[KyleGoetz]

I guarantee you that I was a better English teacher than any native could possibly be. End of story.

First of all, educated teachers do not make mistakes (maybe a few, but they will make far fewer than a non-native, guaranteed). Just like when I'm in professional conversations now, I can shift my speech patterns by situation. I can switch off the "mistakes" (they're not mistakes; they're "non-standard" English) quite easily when I need to.

2. We learn English grammar in junior high the exact same way a non-native learned English. We learn all about subjunctive, prepositional rules, the tenses of English along with their names and such, etc. Just many forget them. The good English teachers will bring this knowledge along with a native's ability to use the rules. I guarantee you I (and almost all educated natives) have an impossibly better grasp of lexical nuance than a non-native can ever hope to achieve.

You may be right on #3. But in this case, the only person equipped to correct it in the way you say is someone who has the same native language as the student. So, in one case, only a native Japanese teaching English would be able to recognize the reasons for a certain error in a Japanese student's English. A Russian-born teaching English would not know why the mistake is being made unless he also speaks Japanese very well.

Regardless, I would never trade any of my four years with native Japanese instructors for anyone else.