关系子句在句中并不是不可或缺的元素，纯粹是增添句子的意义，就算去掉了子句，剩下的句子在文法上仍不受影响。关系子句有两种——非限定子句和限定子句，您务必要了解这两者的区别，因为它会决定该用哪个关系代词来引导子句。在「介词在关系子句中的位置」这一页中，会有详细的说明。
限定子句
当我们在一句话中同时提及多个人事物时，限定子句会表明我们指的是何者。如果限定关系子句从句中移除，就会大大地改变了整句话的意思。限定关系子句不会用逗号或括号来与主要子句隔开。
举例
- The woman who visited me in the hospital was very kind.
- The umbrella that I bought last week is already broken.
- The man who stole my backpack has been arrested.
- The weather that we had this summer was beautiful.
了解如何正确地使用限定关系子句。
非限定子句
非限定子句的作用，是对句中论及的人事物给予额外的信息。如果非限定关系子句从句中移除，句子的整体意思并未改变，顶多少了点信息。非限定子句一般会用逗号或括号来与主要子句隔开。
举例
- The farmer, whose name was Fred, sold us 10 pounds of potatoes.
- Elephants, which are the largest land mammals, live in herds of 10 or more adults.
- The author, who graduated from the same university I did, gave a wonderful presentation.
- My mother, who is 86, lives in Paris.
了解如何正确使用非限定关系子句。