.当先行词是不定代词或被它们修饰时，要用that，不用which，作宾语时可以省去。
例：
All that can be done has been done.
凡能做的事都已经做了。
You can't believe anything she says.
她所说的什么话你都不能相信。
There's nothing you can do to help.
你做什么也无能为力了。
I hope that the little that I've been able to do has been of some use.
我希望我能够做到的那一点点是有些用处的。
This reference book contains much(little)that is useful.
这本参考书中有很多（没有多少）有用的东西。
当先行词被形容词的最高级形式所修饰时，要用that, 不用which，作宾语时可以省去。例：
This is the most expensive watch(that)I've ever owned.
这是我有过的最昂贵的一块表。
This is the best article that has ever been written on this subject.
迄今为止在有关这个题目的文章中，这一篇是最好的。
当先行词被序数词或the very, the only, the last修饰时，要用that，不用which，作宾语时可以省去。例：
This is the first time(that)I've heard her sing.
这是我头一次听她唱歌。
This is the first time I've been here.
这是我头一次来这里。
That's the seventeenth beer(that)you've drunk this evening.
那可是你今天晚上喝的第十七杯啤酒了。
It was the third time he had been in love that year.
这是他那一年第三次谈恋爱了。
This is the very pen he used when he was writing the book.
他写那本书时就是用这支笔。
This is the last thing one can say about it.
决不能这样说。
To invent a lie about my mistake is the last thing I'll do.
说谎话来掩盖我的错误是我最不愿意干的事。
The only thing that matters is to find our way home.
只有一件事最重要，那就是找到回家的路。
当先行词被the same修饰时，用that表示同一件物品，用as表示相类似的一件物品，不能用which。例：
This is the same one as/that you had before.
这跟你以前的一样/这就是你以前的那一个。
I saw the mistake at the(very)same moment that she did.
我与她（正好）同时发现错误。
I bought the same car as yours/that you did.
我买的那辆汽车和你的一模一样。
You've made the same mistakes as last time/that you made last time.
你又犯了上次所犯的同一错误。
当先行词既指人又指物时，只能用that，不能用which。例：
I like the writers and their works that you mentioned the other day.
我喜欢你前几天提到的那些作家和他们的作品。
They were talking about the persons and things that they remembered in the school.
他们在谈论他们能回忆起来在学校里的人和事。 当关系代词在定语从句中作表语是，只能用that，也可以省去，不能用which。例：
China is no longer the country that it was.
中国已不是过去的中国了。
My typewriter is not the machine(that)it was.
我的打字机已不是过去的机器了。
为避免与which重复使用时，要用that。例：
Which is the car that was made in Beijing?
哪辆汽车是北京制造的？
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