主語指人用that who 主語指物用that which
賓語指人用that who whom 或省略 賓語指物用that which 或省略可以舉一個(gè)例子嗎i know the boy () is tall and handsome. 做主語。 從句中，the boy is tall and handsome.
I bought a book ( ) sells well. 做主語。在從句中， thebook sells well.

I know the boy ( ) he is talking with.做賓語，在從句中，he is talking to the boy.
I bought a book ()he wrote. 做賓語，在從句中，he wrote the book.