关系从句有限制性关系从句（英语：restrictive relative clause或defining relative clause）和非限制性关系从句（英语：non-restrictive relative clause或non-defining relative clause）之分。限制性关系从句起限定作用，修饰特定的名词或名词短语；而非限制性关系从句只起补充说明某种信息的作用。例如：
The government which promises to cut taxes will be popular.（限制性关系从句）
The government, which promises to cut taxes, will be popular.（非限制性关系从句）
There were very few passengers who escaped without serious injury. （限制性关系从句）
There were very few passengers, who escaped without serious injury. （非限制性关系从句）
限制性关系从句
从语义上看，限制性关系从句主要起限定作用，修饰特定的人或事物，如果去掉限制性定语从句，整个句子表意会不完整甚至不通顺；从结构上看，限制性关系从句常紧跟先行词，并且同先行词之间一般不加逗号分隔（但不是绝对的）。
限制性关系从句的关系词包括：that, which, who, whom, whose, as, than等。
非限制性关系从句
从语义上看，非限制性关系从句主要起补充说明的作用，有时相当于一个并列分句或状语从句，可以表达原因、目的、结果、条件、让步等意义。例如：
Dr Lee, who had read through the instructions carefully before doing his experiments, did not obtain satisfactory results. （非限制性关系从句表示让步的意义，相当于though Dr Lee had read through the instructions...）
非限制性定语从句的关系词包括：which, who, whom, whose, as等，另外that在非限制性关系从句中并非绝对不可使用。
定语从句例句带翻译：
1.I have some photos that I took in Australia last year.
我有一些去年在澳大利亚拍的照片。
2.The horse that I rode was lazy.
我骑的那匹马很懒。
3.The film that tells an adventure is very interesting.
那部关于探险的电影非常有趣。
4.The writer that I like best is Mark Twain.
我最喜欢的作家是马克吐温。
5.The photo which we liked best in the City and People group was taken by Zhao Min.
在“城市与人”这组照片中，我们最喜爱的是由赵敏拍摄的。
6.A group of photos which show Beijing and Cambridge in England has won the prize for the subject Home and Away.
一组展示北京和英国剑桥的照片赢得了“故士与他乡”这一主题的奖项。
7.He is the boy who won the photo competition last year!
他就是去年赢得摄影比赛的那个男生!
8.Thanks to everyone who entered the competition.
感谢所有参加本次比赛的参赛者。
9.I took some photos for the soldiers whom I met in the army.
我给在部队里碰到的战士们拍了一些照片。
10.Did you go to the talk（that/which）the famous scientist gave at your school ?
你去听那位著名科学家在你们学校做的演讲了吗？
11.Is that the car (that/which) you bought last month?
那是你上个月买的车吗？
12.A friend is a person (who/whom) you know very well and like.
朋友是你十分理解并且喜欢的人。
13.I’d love to have parents who understand me.
我希望父母理解我。
14.He can make lessons that are usually boring interesting.
他能把通常枯燥乏味的课讲得生动有趣。
15.Please forget the girl who has ever hurt you heavily.
请把伤你的心的女孩忘了吧。
16.We will never forget the place where we play basketball together.
我们永远不会忘记我们一起打篮球的地方。
17.This is the room which/that I live in.
这就是我住的房间。
18. This is the reason why he was late.
这就是为什么他迟到的原因。