限制性定语从句是名词词组不可缺少的一个组成部分, 去掉了会造成病句或意义不明确; 非限制性定语从句属于补充说明性质, 去掉了不会影响主要意义, 通常用逗号与它的先行词分开.
The boys who wanted to play football were disappointed when it rained.
The boys, who wanted to play football, were disappointed when it rained.
如果定语从句的先行词是专有名词, 或是带有形容词性物主代词(my, his, etc)或形容词性指示代词(this, that, etc)作限定词, 其后的定语从句通常都是非限制性的:
Mary Smith, who is in the corner, wants to meet you.
Her mother, who had long suffered form arthritis, died last night.
All these books, which have been donated by visiting professors, are to be used by the postgraduates.
在非限制性定语从句中只能用who/whom指人, 用which指物,通常不用that替代.
My father, who had been on a visit to America, returned yesterday.
All the books, which had pictures in them, were sent to the little girl.
如果定语从句中谓语为 there is, 作主语的关系代词也常可省掉:
I must make full use of the time there is left to me and do as much as I can for the people.
This is the fastest train (that) there is to Nanjing.
定语从句的简化: 定语从句与不定式结构, -ing分词结构, -ed分词结构以及无动词分句等有着转换关系.
He was the only one to realize the danger (= who realized the danger).
The woman driving the car (= who was driving the car) indicated that she was going to turn left.
The man injured by the bullet (= who was injured by the bullet) was taken to hospital.
All the women present (= who were present) looked up in alarm.