【定语从句】口诀 （刘老师原创）
定语从句就是句子作定语，它跟其他从句一样（状语从句和名词从句），是英语学习的重点和难点，也是今后英语考试的必考项目，鉴于教材、语法书对此讲解复杂混乱，容易混淆，我特编写了“定语从句口诀”，并辅以【讲解】，除了背诵之外，更要弄懂每条口诀的含义。“定语从句口诀”力争做到：内容全面，归纳条例，思路清晰，例句恰当，科学实用。
定语从句口诀
（一）
定语从句真奇妙，
关系代(副)词来引导，
定语从句分两种，
是否限定看逗号。
（二）
which指物who指人，
地点where时间when,
that人或物均可，
why之前是reason。
（三）
从句当中作宾语，
whom可以代替who,
为了句子更简练，
关系代词可省去。
（四）
关系若是表所属，
whose用法不可无。
除此之外还有啥？
whom, which加of。
（五）
关系代词不一般，
介词经常用在前，
此时不用that, who，
which, whom才安全。
（六）
先行词，很重要，
若指唯一或最高，
还有every & all,
关系代词that好。
先行词后有逗号，
不管指物或指人，
that坚决不能要。
【口诀一 讲解】
定语从句真奇妙，
关系代(副)词来引导，
定语从句分两种，
是否限定看逗号。
该口诀是说，定语从句由关系代词(which, that, who, whom, whose)和关系副词(where, when, why)来引导。请同学们牢记这8个词！
定语从句分“限制性定语从句”和“非限制性定语从句”两种，前者表示与主句关系比较密切。后者与主句的关系不十分密切，只是对先行词作些附加说明而已。如果去掉，主句意思仍然清楚完整。它跟主句之间用逗号分开，翻译时可单独译成一句。例如：
Tom is a boy who often helps others.
（限制性定语从句）
Yesterday I met Tom, who seemed very upset.（非限制性定语从句）
【口诀二 讲解】
which指物who指人，
地点where时间when,
that人或物均可，
why之前是reason。
该口诀是说，选用什么样的关系代词（或关系副词），是由先行词来决定的。先行词是被定语从句所修饰的词。例如：
They planted some trees which didn’t need much water.
The foreigner who visited our school yesterday is from Canada.
This is the house where the old scientist once worked.
I will never forget the day when I joined the Youth League.
Do you know the man that is reading a book over there?
A clock is a machine that can tell time.
I don’t know the reason why he is late.
【注意】当定语从句由关系副词where和when引导时，可以用“介词+which”代替。例如：
This is the house where the old scientist once worked. = This is the house in which the old scientist once worked.
I will never forget the day when I joined the Youth League. = I will never forget the day on which I joined the Youth League.
【注意】并非所有的地点和时间，后面的定语从句都要用where和when引导。如果定语从句中缺少主语或宾语，那么，就要用that或which引导。例如：
This is the house that/which (不用where) my father built last year. （作定语从句的宾语）
Do you still remember the days that/which (不用when) we spent in the countryside? （作定语从句的宾语）
【口诀三 讲解】
从句当中作宾语，
whom可以代替who,
为了句子更简练，
关系代词可省去。
该口诀是说，如果关系代词在定语从句中作了宾语，那么可以省略。此外，whom可以代替who，当然也都可以省略。例如：
The fish (which) I cooked was very delicious.
This is the book (that) I bought last week.
The girl (who) you saw yesterday is my girlfriend.
The girl (whom) you saw yesterday is my girlfriend.
【口诀四 讲解】
关系若是表所属，
whose用法不可无。
除此之外还有啥？
whom, which加of。
该口诀是说，如果先行词与后面的词是所属关系，那么要用whose来引导。例如：
I want to help the children, whose parents died in the earthquake.
We must repair the desks whose legs are broken.
除了用whose表所属关系以外，在whom和which前加of也是一个不错的办法。例如：
上面两句可以分别为：
I want to help the children, whose parents died in the earthquake.
I want to help the children, of whom the parents died in the earthquake.
I want to help the children, the parents of whom died in the earthquake.
We must repair the desks whose legs are broken.
We must repair the desks of which the legs are broken.
We must repair the desks the legs of which are broken.
此外，下面也是一些常见的表达：
She has three daughters, one of whom is a doctor.
She has three daughters, all of whom are beautiful.
She has three daughters, the oldest of whom is called Mary.
【口诀五 讲解】
关系代词不一般，
介词经常用在前，
此时不用that, who，
which, whom才安全。
该口诀是说，介词经常用在关系代词前面。这时关系代词无论指物或者指人，一律不用that, 也不用who，而必须用which或whom。例如：
This is the pen with which she wrote a letter.
The person to whom you talked is his father.
【口诀六 讲解】
先行词，很重要，
若指唯一或最高，
还有every & all,
关系代词that好。
先行词后有逗号，
不管指物或指人，
that坚决不能要。
该口诀是说，先行词表示的是“唯一”或者“最高级”概念，即前面有the only, the first, the last, the best等词修饰的时候，或者先行词含有every (包括everything)和all时，关系代词用that，不用which。例如：
The only thing that we should do is to find our way home.
This is the best film that I have ever seen.
Everything that you have done is helpful to us.
All that she can do is to look after children.
与此相反，如果先行词后有了逗号，即非限制性定语从句。那么，不管指物或指人，that坚决不能用。物用which，人用who或whom。例如：
St. Petersburg, which is a very beautiful city, was once called Leningrad.
Yesterday I met Li Ping, who seemed very busy.
Li Ping is a good student, whom the teacher often praises.
口诀（五）、（六）都讲到了that和which的区别。that和which 是一对孪生兄弟，因为它们都可以指物。因此这两个关系代词经常纠缠不清。它们的区别总结如下：
1. 关系代词之前有介词时，用which不用that。
2. which能引导非限定性定语从句，that则不能。
3. that除了指物还可指人，which则不能。
4. 先行词表示的是“唯一”或者“最高级”概念以及含有every, all时，用that不用which。
5. which可以指整个句子的意思，that则不能。（如：He tired to get out of the trouble, which seemed impossible.
我的回答满意吗？
（刘老师 回答）
追问老师谢谢！ 我的意思是哪些中文句子翻译过来时要用就是说怎么看一句中文翻译是不是要用定语从句
追答【回答】定语从句就是句子作定语，修饰前面的某个词（叫先行词）。例如；
她就是经常帮助我的那个女孩。She is the girl who often helps me.
who often helps me. 是定语从句修饰the girl
她就是我经常帮助的那个女孩。 She is the girl who I often help.
who I often help. 是定语从句修饰the girl
她就是那个对我帮助很大的女孩。 She is the girl whose help is great to me.
whose help is great to me. 是定语从句修饰the girl
这三个定语从句是不同的，所以关系代词也不同。希望你认真阅读我上面的详细讲解吧。
来自：求助得到的回答