Relative clauses add extra information to a sentence by defining a noun. They are usually divided into two types –defining relative clauses and non-defining relative clauses.
定語從句是對句子里的名詞加入額外的信息。一般來講，有兩種類型的定語從句—限定性定語從句和非限定性定語從句。

Non-defining relative clauses
非限定性定語從句

Look at this sentence.
看看這個(gè)例句
? My grandfather, who is 87, goes swimming every day.
‘who is 87’ is a non-defining relative clause. It adds extra information to the sentence. If we take the clause out of the sentence, the sentence still has the same meaning.
‘who is 87’是非限定性定語從句，它給句子增加了額外的信息，如果我們把從句從句子里拿出來，句子的意義沒有受影響。
Look at some more examples.
看看更多的例句
? The film, which stars Tom Carter, is released on Friday.
? My eldest son, whose work takes him all over the world, is in Hong Kong at the moment.
? The car, which can reach speeds of over 300km/ph, costs over $500,000.
Non-defining relative clauses add extra information to sentences.
非限定性定語從句給句子增加了額外的信息。

Defining or non-defining?
限定性還是非限定性？

Remember that defining relative clauses are used to add important information. The sentence would have a different meaning without the defining relative clause.
請記住，限定性定語從句是給句子增加了重要的信息，句子如果沒有了定語從句，其意義不同。
? I’m going to wear the skirt that I bought in London.
The defining relative clause tells us which skirt.
限定性定語從句告訴我們是哪個(gè)短裙。
? The skirt, which is a lovely dark blue colour, only cost ￡10.
The non-defining relative clause doesn’t tell us which skirt – it gives us more information about the skirt.
非限定性定語從句不會告訴我們是哪個(gè)短裙，它給我關(guān)于短裙的更多信息。
Non-defining relative clauses can use most relative pronouns (which, whose etc,) but they CAN’T use ‘that’ and the relative pronoun can never be omitted.
而非限定性定語從句能用關(guān)系代詞which和whose等等，但是它們不能用that，其句子中的關(guān)系代詞也不可以被省略。
? The film, that stars Tom Carter, is released on Friday.
Non-defining relative clauses are more often used in written English than in spoken English. You can tell that a clause is non-defining because it is separated by commas at each end of the clause.
相比起口語，非限定性定語從句更多地用于書寫英語，你能辨認(rèn)出哪個(gè)是非限定從句，因?yàn)榱?xí)慣會在每個(gè)從句的結(jié)尾，用逗號分開。