一、so是副詞，修飾形容詞和副詞；而such是形容詞，修飾名詞。它們後面都接單數可數名詞，但詞序不同。
1.so的詞序為：so+adj.+a（an）+n.
2.such的詞序為：such+a（an）+adj.+n.
它們可以表達同樣的意思，因此它們可以進行同義改寫。
（1）so nice a coat=such a nice coat 這麼漂亮的一件外套
（2）so interesting a book=such an interesting book 那麼有趣的.一本書
注意：後面接複數名詞或不可數名詞時，只能用such，而不能用so。例如：
（1）such beautiful flowers 這麼美麗的花
（2）such clever children 如此聰明的孩子
但是，複數名詞或不可數名詞前有many，few，much，little修飾時，只能用so而不能用such，這是一種固定用法。例如：
（1）so many books 這麼多的書
（2）so few people 這麼少的人
（3）so much money 那麼多的錢
（4）so little milk 那麼少的牛奶
二、和that連用時，意思基本一樣，但句型結構不同。so...that...句型結構為：
1.so+adj.（adv.）+that...
2.so+adj.+a（an）+單數名詞+that...
3.so+many（few）+複數名詞+that...
4.so+much（little）+不可數名詞+that...例如：
（1）This book is so interesting that I have read it three times.這本書如此有趣，我已經看了三遍。
（2）He spoke so fast that we couldn’t understand him.他說得太快，我們都未能聽懂他的話。
（3）It was so hot a day that nobody wanted to do anything.天氣很熱，誰都不想幹活。
（4）There were so many people that we could hardly move on.這麼多人，我們簡直無法繼續往前走。
5.such...that...句型結構為：
6.such+a（an）+adj.+單數名詞+that...
7.such+adj.+複數名詞+that...
8.such+adj.+不可數名詞+that...例如：
（1）She is such a pretty girl that everyone likes her.她是個很可愛的小姑娘，大家都喜歡她。
（2）They are such delicious cakes that I want to eat another two.這麼可口的蛋糕，我還想再吃兩塊。
（3）It is such sweet milk that we all want to drink it.這麼香的牛奶，我們都想喝。
注意：由於so和such後跟單數可數名詞時，可以換用，同樣so...that...與such...that...也可以進行同義句改寫。如上文中：This book is so interesting that I have read it three times.可改寫成：This is such an interesting book that I have read it three times. It was so hot a day that nobody wanted to do anything.可改寫成：It was such a hot day that nobody wanted to do anything.
三、so與that可以直接構成片語so that，引導目的狀語從句和結果狀語從句，表示“以便、以至”的意思。例如：
He worked hard so that he could pass theexams.為了能透過考試，他學習很認真。（但such沒有這種用法。）