he doesn't know where the post office is

问题描述:

he doesn't know where the post office is
is 后怎么可以跟where啊?

"where"是副词,在这里做宾语从句中is的表语.宾语从句中,the post office 是主语,单数第三人称,系动词当然是is啦.
正常的陈述语序应该是He doesn't know the post office is where.(他不知道邮局在哪)