先看几道考题，能做对就不用往下看了：
The old town has narrow streets and small houses _____are built close to each other.
A．who B．that C．what D．They
The TV show Running Man is popular recently.
— Yes. It is the only program ________ I watched this month.
A．who B．when C．where D．that
如果您对上面的问题还有疑惑，请仔细阅读本文。题目的答案及解析会在文章中逐步给出。
可以用that：独立的who which都可以用that代替。
Here is the pen which（that）you lost yesterday.
The man who（that）talked to you just now is an engineer.
不能用that：非限定性定语从句不能用that；介词后面的which不可以用that替代
The chair in which she sat is made of wood
她坐的椅子是用木头做的。( which作介词的宾语)
Mo yan was awarded the nobelprize in literature , which made us happy
莫言被授子诺贝尔文学奖,这使我们非常高兴。( which引导非限制性定语从句)
“必须”用that
先行词是：all，few, little, much, something, nothing, anything等。
All that we have to do is to practise English.
先行词被序数词或形容词最高级所修饰，如
The first letter that I got from him will be kept.
先行词被all, any, every, each, few, little, no, some等修饰：
I've eaten up all the food that you gave me.
先行词被the only, the very, the same, the last修饰时如
He is the only person that I want to talk with.
先行词既有人又有物时：
They talked about persons and things that they met.
当句中已有who时 ， 为避免重复 ， 如 ： Who is the man that is giving us the class?
如何理解与记忆
which的英文解释：used for asking for a specific choice from a limitednumber of possibilities.
which的两个条件：两种以上、有限种可能。不符合这两个条件的就用that。如上所述前四种情况，要么唯一，要么全部，要么范围模糊不定，所以不能用which，只能用that.
下面解析前文考题：
The old town has narrow streets and small houses _____are built close to each other.
A．who B．that C．what D．They
【答案】B 此处用which也正确
The TV show Running Man is popular recently.
— Yes. It is the only program ________ I watched this month.
A．who B．when C．where D．that
【答案】D 此处必须用that