在定语从句中又可分为两大类定语从句，即限制性定语从句和非限制性定语从句。
① 限制性定语从句是先行词在意义上不可缺少的定语，如果去掉的话，主句的意思就不完整，意义就表述不明。这种句型一般定语从句紧接先行词，如：I was the only person in my office who was invited
② 非限制性定语从句。它与主句的关系不十分密切，只是对其附加说明，也就是讲即便去掉定语从句，句意也不受影响，仍然清晰明了。这样的定语从句要在它和主句之间加一逗号分开。且关系代词不引导这种非限制性定语从句，如： Abraham Lincoln， who led the United States through these years， was shot on April 14， 1865 at a theatre in washington D. C. 又如：Galileo lived in the city of Pisa， where there is a leaning tower about 180 feet high
(二) 正误辨析
[误] I won't tell you the name of the person who teach me English
[正] I won't tell you the name of the person who teaches me English
[析] 在定语从句中，关系代词作主语时，从它本身看不出其数的形式，这时要由它的先行词决定。这里who 应由 the person 单数决定，应该用单数谓语动词。又如：I who am a student want to find a spare time job这里的 who 应与 I 是一致的，所以其谓语动词应该用am。
[误] We talked about the things and the people who we met during the Second World War
[正] We talked about the things and the people that we met during the Second World War
[析] 这里的关系代词不要用 who， 因为其先行词有两个一个是 things (物)，而另一个是people (人)，这时既不可用 who， 又不可用 which， 因前者只能用于先行词是人的情况下，而后者则用于先行词是物的情况下，所以只能用 that， 因为它的先行词既可以是人又可以是物。
[误] The book， that I bought yesterday， was very good
[正] The book， which I bought yesterday， was very good
[析] 先行词与定语从句被逗号分割开来时，即作为非限制性定语从句。在非限制性定语从句中 which， when， who， whom， where， when， whose 等都可以和限制性定语从句中的作用一样，而独有 that 不易用于非限制性定语从句。
[误] The dictionary which I lent it yesterday is a very useful tool
[正] The dictionary which I lent yesterday is a very useful tool
[析] 关系代词在定语从句中是要起语法作用的，它不是作主语就是作宾语。虽然在作宾语时它的位置由原来的宾语位置移到了句首，但它的作用依然存在，而且在原宾语位置上不能再出现宾语。
[误] The teacher I want to learn English from is the one which comes from America
[正] The teacher I want to learn English from is the one who comes from America
[析] the one， anyone， those 作代词并且是指某人、物时，其关系代词不能用 which 应用 who。
[误] This is the room in that the old man lives
[正] This is the room in which the old man lives
[正] This is the room which the old man lives in
[正] This is the room that the old man lives in
[析] that 不能紧跟在介词后作介词宾语，但如果介词不前置仍放于句尾，则可用 that 作引导词，而且可以省略。如： This is the room the old man lives in
[误] I can do everything which is good for you
[正] I can do everything that is good for you
[析] 在先行词是 all， much， little， something， everything， anything， nothing， none， the one 等不定代词时，虽然它们指的是物体，但不要用 which 而用 that 作定语从句的引导词。
[误] The only thing which the students can do is studying hard
[正] The only thing that the students can do is studying hard
[析] 在先行词前有 only， any， few， little， no， all， one of 等词修饰时，虽然先行词指的是物，也不要用 which 作关系代词，而要用 that。
[误] This is the first American film which I've ever seen
[正] This is the first American film that I've ever seen
[析] 在先行词是序数词，或由序数词修饰时，其关系代词不可用 which 这样的用法还有在形容词最高级修饰的先行词之后，如： This is the best book that I have ever seen
[误] He is from Africa, that we can see from the colour of the skin
[正] He is from Africa, as we can see from the colour of the skin
[析] 当 as 或 which 引导非限制性定语从句时，它可能没有明确的先行词，它们所指代的是前面整个句子。如例题应译为他是从非洲来这个事情是可以从其肤色上看出的。
