so後面都加形容詞,that後面加句子,as後面加名詞.
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引導定語從句時,可指人也可指物,通常與the same, such, so等連用,形式爲:the same … as, such … as, as … as, so … as。
as在定語從句中可作主語、賓語、表語等。如:
She knew he felt just the same as she did. 她知道他的感覺與她自己的一樣。(as代替事,作賓語)
I lend you such books as will interest you. 我借給你使你感興趣的書。(as代替物,作主語)
as引導的定語從句表示其內容與主句所講的事物“相似”,指同類事物that引導的定語從句表示其內容與主句所講的事物是“同一個”,指同一事物。如:
This is the same pen as I lost yesterday. 這和我昨天丟的筆一樣。(類似的筆)
This is the same pen that I lost yesterday. 這支筆就是我昨天丟的那支。(同一支筆)
so…as在定語從句中的用法是:so 位於定語從句先行詞前面,修飾先行詞,並且先行詞在從句中作主語或賓語,關係代詞用as, 例如:
He is so clever a boy as all of us like very much.他是一個我們大家都非常喜歡的聰明的男孩(本句中,先行詞a boy 在定語從句all of us like very much中作like的賓語,而且前面有so 修飾,所以關係代詞用了as)