1. 用于引导限制性定语从句，主要与such, as, the same连用，其意为“像 ……的”“凡是……的”“……一类的人 (物)”。如：
He’s not such a fool as he looks.
He is not such a man as would leave his work half done.
You must show my wife the same respect as you show me.
He smokes as expensive a cigarette as he can afford.
2. 用于引导非限制性定语从句，根据情况可位于主句之前或之后，有时也可插在主句中间，其 意为“这件事”“这一点”。如：
He is late, as is often the case.
As is usual with children, they soon got tired.
David, as you know, has not been well lately.