主语指人用that who 主语指物用that which
宾语指人用that who whom 或省略 宾语指物用that which 或省略可以举一个例子吗i know the boy () is tall and handsome. 做主语。 从句中，the boy is tall and handsome.
I bought a book ( ) sells well. 做主语。在从句中， thebook sells well.
I know the boy ( ) he is talking with.做宾语，在从句中，he is talking to the boy.
I bought a book ()he wrote. 做宾语，在从句中，he wrote the book.