Why native speakers can be the worst teachers.

Why native speakers can be the worst teachers.

I have seen many native speakers trying to teach their own languages to foreigners.
But in my opinion most of them are quite bad.
I have seen English speakers trying to teach English, Japanese people trying to teach Japanese and Brazilians trying to teach Portuguese.
Why are most of them so bad at teaching?
Some reasons:
- they ignore textbooks and grammar books
- they ignore the cultural background of their students
- they rely on their own subjective view of the language
- they give too much emphasis on slangs and idioms that are not universally accepted
- they have no formal training as teachers
- they give too much emphasis on informal instead of formal language
- they give too much emphasis on spoken instead of written language
- they don't try to learn the language of their students
- they assume that non-verbal elements like gestures are universal

Why do so many native speakers just ignore textbooks and grammar books? It seems that they think they can teach relying only on their guts. Many textbooks were written by professional educators and are extremely helpful for teaching a language.

I am not saying that native speakers should not teach. I think it is good that some people are willing to teach foreigners. Native speakers are ideal to practice conversation and pronunciation. Most students want to make friends with native speakers, to exchange letters, to talk.

But for grammar explanations and for the learning of the language in general, most students are better off with a book written by a fellow countryman.