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.
定语从句是对句子里的名词加入额外的信息。一般来讲，有两种类型的定语从句—限定性定语从句和非限定性定语从句。

Non-defining relative clauses
非限定性定语从句

Look at this sentence.
看看这个例句
• 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’是非限定性定语从句，它给句子增加了额外的信息，如果我们把从句从句子里拿出来，句子的意义没有受影响。
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.
非限定性定语从句给句子增加了额外的信息。

Defining or non-defining?
限定性还是非限定性？

Remember that defining relative clauses are used to add important information. The sentence would have a different meaning without the defining relative clause.
请记住，限定性定语从句是给句子增加了重要的信息，句子如果没有了定语从句，其意义不同。
• I’m going to wear the skirt that I bought in London.
The defining relative clause tells us which skirt.
限定性定语从句告诉我们是哪个短裙。
• 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.
非限定性定语从句不会告诉我们是哪个短裙，它给我关于短裙的更多信息。
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.
而非限定性定语从句能用关系代词which和whose等等，但是它们不能用that，其句子中的关系代词也不可以被省略。
• 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.
相比起口语，非限定性定语从句更多地用于书写英语，你能辨认出哪个是非限定从句，因为习惯会在每个从句的结尾，用逗号分开。