不要担心,看你有英语底子.关系代词顾名思义是一个代词,代替先行词.所以只需看先行词在定语从句中的位置即可明白.用that不用which
1）先行词是最高级或他前面有形容词最高级时
This is the best --that-- has been used against air pollution in cities.
English is the most difficult subject --that-- you will learn during these days.
2)先行词是序数词或它前面有一个序数词时
He is the last person --that-- i want to see.
3)主句中有疑问句
Which is the pen --that-- you lost?
4)先行词既有人又有物时
The bike and its rider --that-- had run over an old man were taken to the police station.
5)先行词是 all, much, little ,something,everything,anything,nothing,the one时
6）先行词前有 only,any,few,little,no ,all,one of,the same, the very等词装饰时
7）有2个定语从句,其中一个关系代词已用which,另一个应用that.
Tom built up a factory which produced things that had never been seen before.