在定语从句中
作主语,先行词是人用who/that
先行词是物用which/that
eg.一个男孩a boy
一个能讲三国语言的男孩the boy who/that can speak three foreign languages
一扇门a door
一扇木制的门the door which/taht is made of wood
作宾语,先行词是人用whom/that(who)
先行词是物用which/that
eg.一个女孩a girl
一个警察正在找的女孩the girl whom/that the police are looking for
一把钥匙a key
一把我昨天弄丢了的钥匙the key which/that I lost yesterday
eg.I saw a tree.Its leaves were black.
→I saw a tree whose leaves were black.
=I saw a tree of which the leaves were black.
先行词指人：
whose + n.=the + n.+ of whom(或of whom + the + n.)
先行词指物：
whose + n.=the + n.+ of which（或of which.+ the + n.)
只能用that的7种情况：
1.先行词是不定代词
2.先行词被all,any,every,few,little等不定代词所修饰
3.先行词是序数词或被形容词的最高级所修饰
4.先行词被the only,the very(就是那个）所修饰
5.先行词既有人又有物时
6.当句子已有who时（为避免重复）
7.关系代词在从句中作表语时
不能用that的2种情况：
1.引导非限定性定语从句时,一般用which,不用that
2.关系代词直接放在介词后面时,要用which/whom
