定语从句什么时候不能用that

1.被不定代词修饰时
2.有比较级，最高级修饰时
3.前面已经有用that
4.非限制性定语从句就不能用that

关于代词that和which的用法区别：
在定语从句中，当先行词是指物时，关系代词which可以由that代替，如果which不在介词之后，也可省略。在日常用语或口语中，which不作主语时可以省略。例：
Did you take away the book(which/that）I showed you yesterday?
你把我昨天给你看的那本书拿走了吗？
The hotel at which we stayed was both cheap and comfortable.
我们住的那家旅馆既便宜又舒适。
This is certainly not the book for which I paid./This is certainly not the book I paid for.
这肯定不是我付钱买的那本书。

但两者之间还有一定的区别，必须予以足够的重视。

一.that仅用在限制性定语从句中，通常不用逗号隔开，作宾语或介词宾语时，that 可以省去。例：
The letter that came this morning is from my father.
今晨收到的那封信是我父亲寄来的。
The watch(that)you gave me keeps perfect time.
你给我的那块手表走得准极了。
The photographs(that) you're looking at were taken by my brother.
你正看的这些相片是我弟弟照的。

二.which既可用在限制性定语从句中，作宾语或介词宾语时，which可以省去，也可用在非限制性定语从句中。例：
Did you see the letter that/which came today?
今天来的信你看到了没有？
This is the one of which I'm speaking./This is the one I'm speaking of.
这就是我说的那一个。
He changed his mind, which made me angry.
他改变了主意，使我很生气。
He admires Mrs.Brown, which surprises me.
他钦佩布郎太太，这使我感到惊奇。

三.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。

四.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。

五.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。

六.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。

七.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。

八.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。

九.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？

十.关系代词直接放在介词后面时，要用which。例：
Read the passage to which I referred in my talk.
读一下我在演讲中提到过的那段文章。
His new car, for which he paid ￡7000, has already had to be repaired.
他花700英镑买来的那辆新车已经不得不修理了。

十一.which还可以用作关系限定词，后跟名词，that则不能这样用。例：
He changed his mind for the second time, after which I refused ever to go out with him again.
他第二次变卦，以后我再也不跟他出去了。
The postman comes at 6.30 in the morning, at which time I am usually fast asleep.
邮递员早晨六点半就到，而这时我通常还熟睡着呢。
He may come, in which case I'll ask him.
他也许会来，来了我就问他。
There are 2 left, one of which is almost finished and the other of which is not quite.
剩下两件事，一件快做完了，另一件还有得做。

十二.引导非限制性定词从句时，一般要用which，不用that。例：
Football, which is a popular game, is played all over the wofld.
全世界都踢足球，它是一项很受欢迎的运动。
She spent all evening talking about her latest book, which none of us had ever heard of.
她花了整个晚上谈她最近的新书，关于这本书，我们没有一个人听说过。