一、so是副词，修饰形容词和副词；而such是形容词，修饰名词。它们后面都接单数可数名词，但词序不同。
1.so的词序为：so+adj.+a（an）+n.
2.such的词序为：such+a（an）+adj.+n.
它们可以表达同样的意思，因此它们可以进行同义改写。
（1）so nice a coat=such a nice coat 这么漂亮的一件外套
（2）so interesting a book=such an interesting book 那么有趣的一本书
注意：后面接复数名词或不可数名词时，只能用such，而不能用so。例如：
（1）such beautiful flowers 这么美丽的花
（2）such clever children 如此聪明的孩子
但是，复数名词或不可数名词前有many，few，much，little修饰时，只能用so而不能用such，这是一种固定用法。例如：
（1）so many books 这么多的书
（2）so few people 这么少的人
（3）so much money 那么多的钱
（4）so little milk 那么少的牛奶
二、和that连用时，意思基本一样，但句型结构不同。so...that...句型结构为：
1.so+adj.（adv.）+that...
2.so+adj.+a（an）+单数名词+that...
3.so+many（few）+复数名词+that...
4.so+much（little）+不可数名词+that...例如：
（1）This book is so interesting that I have read it three times.这本书如此有趣，我已经看了三遍。
（2）He spoke so fast that we couldn’t understand him.他说得太快，我们都未能听懂他的话。
（3）It was so hot a day that nobody wanted to do anything.天气很热，谁都不想干活。
（4）There were so many people that we could hardly move on.这么多人，我们简直无法继续往前走。
5.such...that...句型结构为：
6.such+a（an）+adj.+单数名词+that...
7.such+adj.+复数名词+that...
8.such+adj.+不可数名词+that...例如：
（1）She is such a pretty girl that everyone likes her.她是个很可爱的小姑娘，大家都喜欢她。
（2）They are such delicious cakes that I want to eat another two.这么可口的蛋糕，我还想再吃两块。
（3）It is such sweet milk that we all want to drink it.这么香的牛奶，我们都想喝。
注意：由于so和such后跟单数可数名词时，可以换用，同样so...that...与such...that...也可以进行同义句改写。如上文中：This book is so interesting that I have read it three times.可改写成：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.可改写成：It was such a hot day that nobody wanted to do anything.
三、so与that可以直接构成词组so that，引导目的状语从句和结果状语从句，表示“以便、以至”的意思。例如：
He worked hard so that he could pass theexams.为了能通过考试，他学习很认真。（但such没有这种用法。）