this is the house (where)we uesd to sleep
=this is the house (in which)we uesd to sleep
句子还原:
we uesd to sleep in the house
house在动词后面,所以是宾语
I found a man(whom/who) is standing under that tree
还原:a man is standing under that tree
a man是宾语,所以用whom/who或者省略
若a man who is standing under that tree .
那么man作主语,只能用who
Understand?