relative clauses定语从句
defining relative clauses 限制性定语从句
non-defining relative clauses非限制性定语从句
relative pronouns 关系代词
来源：bbc.englishlearning.com
一、Meaning and use
Relative clauses are used to give additional information about a noun, such as a person, place or thing. Relative pronouns introduce a relative clause. They include who for people, that and which for things, when for time, and whose to show possession. Relative clauses belong to one of two categories: defining relative clauses and non-defining relative clauses.
定语从句提供有了有关名词的附加信息，例如提供了人，地点或事物的附加信息。 关系代词引入从句。 关系代词包括用于人的who，用于事情/物品的that和which，when用于时间，whose展示归属。 定语从句属于以下两类之一：限制性定语从句和非限制性定语从句。
1. Defining relative clauses add essential information to a sentence.
定语从句为句子（主句）添加必要的信息。
The woman who found my wallet handed it in to reception.
The student whose dog has run away has gone to look for it.
I remember the day when we first met.
These are the earrings that my mother gave me.
These clauses give essential information about the subject of the sentence.
They define the person, time or thing that we are talking about. If we remove the clause, the sentence does not make sense.
这些从句提供了有关句子主题的必要信息。他们定义了我们正在谈论的人，时间或事物。如果删除该子句，则该句子没有意义。
2. Non-defining relative clauses add extra information to a noun or noun phrase.
非限制性定语从句为名词或名词短语添加了额外的信息
My friend’s birthday, which was last weekend, was great fun.
My current girlfriend, who I love very much, calls me every night.
This extra information is not essential. If we remove the clause, the sentence still makes sense.This type of clause is more common in written English.
这些额外信息不是必需的。 如果我们删除该非限制性定语从句，整个句子的意思依然完整。
例如：
My friend’s birthday, which was last weekend,was great fun.
我朋友的上周末生日宴会非常有趣。
把非限制性定语从句删除后，句子的意思依然完整。
My friend’s birthday was great fun.
我朋友的生日宴会非常有趣。
二、Form形式
Defining relative clauses are made with noun + relative pronoun + rest of clause.
限制性定语从句：名词+关系代词+剩下的从句
A kangaroo is an animal which lives in Australia.
The man who came for lunch was my uncle.
Winter is a time when it sometimes snows.
Non-defining relative clauses are made in the same way.
An important difference, however, between both types of clause, is the use of punctuation. With non-defining relative clauses, we separate the clause with commas. We cannot use that in this type of clause.
但是，这两种子句之间的重要区别是标点的使用。 对于非限制性的定语从句，我们用逗号分隔该从句。 我们不能在这种定语从句中使用关系代词that。
My favourite food, which used to be Italian, is now Japanese.
Rachel, who we met yesterday, lives in this neighbourhood.
My car, which I bought seven years ago, needs replacing.
This shirt, which I bought last weekend, cost £50.
My best friend, who I met at university, is coming for dinner.
Take note: replacing the relative pronoun替换关系代词
informal communication, relative pronouns, such as who and when, are commonly replaced with that in defining relative clauses.
在非正式交流中，关系代词（例如，who和when）在定义相对从句时通常用that替换。
The woman who called last night was very polite.
=The woman that called last night was very polite.
Do you remember the time when you first met?
=Do you remember the timethat you first met?
Take note: leaving out the relative pronoun省略关系代词
When using defining relative clauses in informal speech and writing, the relative pronoun can be left out completely if it refers to the object of the relative clause.
在非正式演讲和写作中使用限制性定语从句时，如果关系代词指代从句的宾语，则可以完全省去。
This is the shirt that I bought.
解释：that代指shirt，在从句中做bought的宾语，常常可以省略that。
This is the shirt I bought.
The girl who I like isn’t here yet.
The girl I like isn’t here yet.
特别注意
In non-defining relative clauses, the relative pronoun cannot be left out.
非限制性定语从句中，关系代词不可以省略。
Take note: spoken English 英语口语
The relative pronoun who is used when referring to people. However, in formal written and spoken English, if the pronoun refers to the object of the clause, we use whom instead.
指代人时使用关系代词who。 但是，在正式的书面和口语英语中，如果关系代词指代从句的宾语（即关系代词做宾语），我们将改用whom（而不是who）。
注意：whom是who的宾格形式
My German teacher, whom I really admired, retired last year.
The woman whom I called this morning was my secretary.
备注：在中国的考试中，你不必考虑是否正式，如果同时出现who和whom,而关系代词又是做宾语，就选择最地道的的whom（虽然who也可以）
三、Non-defining relative clauses非限制性定语从句
5 things about non-defining relative clauses
关于非限制性定语从句的5件事情
- You can remove a non-defining relative clause and the sentence will still make sense. It adds extra information about the noun.
移除从句后，整个句子的意思依然通顺。因为从句添加的是附加的信息。
Their dog, which spent its first year living on the streets, didn't like sleeping in the house.Their dog didn't like sleeping in the house.
- You can always identify a non-defining relative clause because there is a comma after the noun it's describing. There will always be a comma at the end of the non-defining clause as well unless it finishes the sentence.
您始终可以辨认非限制性定语从句，因为在其所描述的名词之后有一个逗号。
在从句的末尾也始终会有一个逗号，除非该从句语句结束。
（意思就是如果从句在中间，那么前后都有一个逗号，如果从句在最后，那么从句前有个逗号，从句后是句号）
Her hair, which was naturally brown,was dyed blonde.【which 前有个逗号，从句后也有个逗号】
She had blonde hair, which was dyed.【which前有个逗号，从句后是句号，因为句子结束了】
- Non-defining relative clauses always need the correct relative pronoun (which, who, whom). In defining relative clauses, these pronouns are often replaced with 'that'. It's also not possible to leave the pronoun out of a non-defining relative clause.
非限制性定语从句始终需要正确的相对代词（which，who，whom）。 在限制性定语从句中，这些代词通常被“ that”代替。 在非限制定语从句中关系代词不能被省略。
Correct: Their car, which was 20 years old, had begun to fall apart.
Wrong: Their car, that was 20 years old, had begun to fall apart.
Wrong: Their car, was 20 years old, had begun to fall apart.
- Non-defining relative clauses can add extra information about the quantity of objects described. We do this by using expressions such as many, some, all, a few, half followed by the correct relative pronoun.
非限制性定语从句的关系代词前可以加many of, some of , all of, a few of, half of之类的表达。
The people, half of whom had never voted before, elected a new leader.
The patients, many ofwhom smoked, were suffering from lung conditions.
- You can comment on a whole sentence using non-defining relative clauses.
你可以用非限制性定语从句评论整个句子。
The pigeons, which were all over the town square, left a terrible mess, which was disgusting.
这个句子中两个which引导的定语从句，第一个which指代pigeons，第二个which指代前面整个句子。
The children, who were having a wonderful time, didn't want to go home, which didn't surprise me.