1、so可用来修饰形容词和副词,such修饰名词.
He is such an old man.
He is so old (a man).
2、对比用法：so+形容词+冠词+名词 so important an event
such+冠词+形容词+名词 such an important event
通常是区别so ...that,such...that用法.
3、名词前有表示“多、少”意思的manymuchfewlittle时,只能用so,不能用such.
There are so many flowers in our school garden.
Ihave so little money that I can't lend you any.
另：当little表示“小”时,只能用such 不能用so 修饰