在非限制定语从句中，什么时候as能替换which?

as和which都可以代表整个句子，但二者的用法不同：as引导的定语从句可置于主句之后或主句之前，which引导的定语从句只能置于从句之后。
eg：He is a foreigner,which I know from his accent.
He is a foreigner,as I know from his accent.
As I know from his accent,he is a foreigner.
上面三句话表达意思基本一致，也体现了as和which的用法。他是一个外国人，我是从他的口音得知的。

好象是as不能打头的，唉，太细的都忘了