however和as 引导让步状语从句时的区别：
however引导让步状语从句时,相当于no matter how,后面跟形容词或副词；as引导让步状语从句时,用倒装语序,即把作状语的副词或作表语的形容词或名词提前,而且如果作表语的是单数名词,前面不加冠词,也可以把谓语动词部分的实义动词提前.
〔误〕 As hard he works,he can’t catch up with his classmates.
〔正〕 However hard he works,he can’t catch up with his classmates.
〔析〕见上述说明.