As可以担当句子成分,As you know(as作 know宾语), 而that只是一个连词,
As还有正如……(及前面所提到的事情)的意思,
如:
as you know.一般用于非限定性定语从句,'正如大家知道", 可以放在句首.
that作连接代词,起引导作用,代替先行词在从句中作成分,
You haven't payed me back the money that you borrowed from me.
that 代替MONEY,作borrow 的宾语
而As可以表示前面整个句子的意思。
As is known to all , the Dragon Boat Day falls on lunar clendar on May , the 5th.
as 代替后边整个句子