如何辨别宾语从句和主语从句

问题描述:

如何辨别宾语从句和主语从句
如何辨别
I don't know if you can help me.
I didn't know that you would come until Friday.
哪句是宾语从句哪句是主语从句?
为什么?
书上写I don't know if you can help me.是宾语从句,而I didn't know that you would come until Friday.是主语从句.

书本错误,两个都是宾语从句