Well, I can teach English... I did teach English to one of my
former students.
However, I'm not that proficient in teaching English... being an
engineering major, I'm better at teaching Maths and Science. If
anything, I never actually did any revision for either my O Level
English or my A Level GP. Not trying to boast, but I still got A1
for both of them. 
In the end, the way I see it, the only way you can do well in the
languages is to actually immerse yourself in them. Read lots of
good books, and take the effort to converse in standard
English.
One of my former students insisted (against my recommendations) on
reading only English-dubbed Japanese manga and typing all his SMS
and MSN messages in shorthand notation. In the end, his writings
were full of things like '2mr', '4gt', 'tonite', and so on.
Not surprisingly, he ended up either failing or barely passing all
his English tests. 
