1.Mr Wang taught them Japanese last year.
teach sb. sth. 教某人东西 不能用代词
2.You'd better speak as much English as you can.
as many/much as 尽可能多的 English是不可数名词,所以用much 这里没有比较的意思
3.If you pay more attention to your spelling, you will make fewer mistakes.
mistakes是可数名词,这里用few来表示
little-less-least 修饰不可数N.
few-fewer-fewest 修饰可数N
请楼主验收