告诉你吧,as和that都可以引导定语从句.定语从句的关键点是抓“先行词”
as+从句 这个as引导非限制行定语从句,as指代前面主句的整个“句子”.
He is an englishman,as I know from his accent.
that+从句 这个that指代前面主句的"一个成分".
the girl that you saw is my girlfriend.这个that指代the girl.
当然 as也可以引导 限制性定语从句,主要在这样的短语中such...as the same...as
this is the same pen as I lost last week.
there is no such people as he is.