1）介词后面的关系词不能省略。
2）that前不能有介词。
3） 某些在从句中充当时间，地点或原因状语的"介词+关系词"结构可以同关系副词when 和where 互换。
This is the house in which I lived two years ago.
This is the house where I lived two years ago.
Do you remember the day on which you joined our club?
Do you remember the day when you joined our club?
先行词和关系词二合一：
1)Whoever spits in public will be punished here.(Whoever 可以用 anyone who 代替)
2)The parents will use what they have to send their son to technical school.
(what 可以用all that代替)
