as和which都是引导定语从句的关系代词，它们在引导非限制性定语从句的时候，as位置比较灵活，可以放在句首，也可以放在句中和句末，而which引导非限制性定语从句时是只能放在句中或句末，不能放在句首，如：As is known to us all，Taiwan Island belongs to China since ancient times. 而：Taiwan Island belongs to China since ancient times，which is known to us all.
