A such是形容词，用于形容词+名词结构之前：
They had such a fierce dog that no one dared to go near their house.
他们有一条如此凶猛的`狗，以致没人敢靠近他们家。
He spoke for such a long time that people began to fall asleep.
他说了这么长时间，以致在座的人都犯困了。
B so是副词，用于副词和不带名词的形容词之前：
The snow fell so fast that our footsteps were soon covered up.
雪下得这么快，以致我们的脚印很快就被雪盖住了。
His speech went on for so long that people began to fall asleep.
他的讲话这么长，以致在座的人都开始犯困了。
Their dog was so fierce that no one dared come near it.
他们的狗太凶猛了，所以没人敢靠近它。
但such不能用于much和many之前，所以so可用于后跟名词的much和many之前：
There was so much dust that we couldnt see what was happening.
灰尘太大了，使得我们看不清发生了什么事。
So many people complained that they took the programme off.
抱怨的人太多，所以他们取消了那个节目。