5. 当先行词指人时that一般可以与who 通用；当先行词指物时，that一般可以与which通用。但是当先行词符合下列条件之一时，只能用that.
⑴ 当先行词指物为不定代词时，只用that引导定语从句。
I've forgotten
everything that I learnt at school. 我在学校学的东西我全忘记了。
⑵ 当先行词是最高级或被最高级所修饰时，只能用that引导定语从句。
This is the best
way that we can raise money. 这是我们可以筹钱的最好方法。
⑶ 当先行词是序数词或被序数词所修饰时，只能用that引导定语从句。
He is the first
person that I met this morning. 他是我今天早上见到的第一个人。
⑷ 当先行词被only, very, just, last, one of,
all no, little, few, any等词修饰时，只能用that引导定语从句。
It is one of the
beautiful views that I have seen. 这是我所见到的最美的风景之一。
⑸ 当先行词既有人又有物时，只能用that引导定语从句。
We talked the things and persons that we were interested in.
我们谈论我们感兴趣的人和事。
⑹ 如果有两个从句，其中一个用了who (which)来引导，则另一个用that以避免重复。
He built up a
factory which produced things that had never been seen before.
他建造了工厂要生产一些我们以前没见过的东西。
⑺ 当先行词被the only,
the very, the some, the last等词修饰时，只能用that引导定语从句。
This is the last place that I want to visit. 这是我想参观的最后一个地方。