两个句子不是都修饰主句吗吗，1说只能用as，而2怎么用which ？

两个句子不是都修饰主句吗吗，1说只能用as，而2怎么用which ？
学习时看到看到:
1.当非限制性定语从句修饰整个主句时，其关系代词只能用as ，不能用which。此时非限制性定语从句可置于主句之前、之后或之中。
Mary was late for school,as often happened.
2. which指代整个主句。
In the presence of so many people he was little tense, which was understandable.
在那么多人面前他有点紧张，这是可以理解的。