先行词表示物时，关系代词只能用that而不用which的情况如下：
1) 不定代词（all, much, something, everything, anything, nothing, none, the one）作先行词时；
e.g. There is nothing that can be done by us.
2) 先行词被only, any, few, little, no, all, one of, just, very, much等词修饰时；
e.g. The only thing that I can do is to wait. The pen is the only thing that I have.
3) 先行词是序数词或被序数词修饰时；
e.g. This is the first time that I visit Beijing.
4) 先行词是形容词最高级或被形容词最高级修饰时；
e.g. The most important thing that I did is being a good father.
5) 先行词既有人又有物时；先行词既有时间又有地点时；
e.g. The police are looking over the man and his dog that were killed by the tiger.
The time and the place that we will hold the meeting has not been decided yet.
6) 先行词是数词时；
e.g. I bought 3 pens yesterday, now you can find 2 that have broken.
7) 如有两个从句，其中一个关系代词已用which，另一个关系代词宜用that。
e.g. He built a factory which produced things that had never been seen before.
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