so后面都加形容词,that后面加句子,as后面加名词.
如:The biscuits are so delicious that I want to eat more. 
The food is not so good as those we ate in Bill's home.
As引导定语从句时,可指人也可指物,通常与the same, such, so等连用,形式为:the same … as, such … as, as … as, so … as。
as在定语从句中可作主语、宾语、表语等。如:
She knew he felt just the same as she did. 她知道他的感觉与她自己的一样。(as代替事,作宾语)
I lend you such books as will interest you. 我借给你使你感兴趣的书。(as代替物,作主语)
as引导的定语从句表示其内容与主句所讲的事物“相似”,指同类事物that引导的定语从句表示其内容与主句所讲的事物是“同一个”,指同一事物。如:
This is the same pen as I lost yesterday. 这和我昨天丢的笔一样。(类似的笔)
This is the same pen that I lost yesterday. 这支笔就是我昨天丢的那支。(同一支笔)
so …as在定语从句中应用
so…as在定语从句中的用法是:so 位于定语从句先行词前面,修饰先行词,并且先行词在从句中作主语或宾语,关系代词用as, 例如:
He is so clever a boy as all of us like very much.他是一个我们大家都非常喜欢的聪明的男孩(本句中,先行词a boy 在定语从句all of us like very much中作like的宾语,而且前面有so 修饰,所以关系代词用了as)