as在引导非限制性定语从句时的用法几乎和which差不多,但有两点需注意,
1）as可位于句首,which不行
2）as往往表顺其自然的事情,也就是肯定句或表示肯定意思的句子,如：As we had expected,he married her.
He married her,which we hadn't expected.
as在引导限制性定语从句时也主要是两种用法
1）such+先行词+as的从句
2）the same+先行词+as的从句
关于第二条,需要注意若是同类用as,若是同一用that如
He bought the same pen as I had.
He found the same pen that he lost.