定语从句有限制性和非限制性两种.限制性定语从句是先行词不可缺少的部分,去掉它主句意思往往不明确；非限制性定语从句是先行词的附加说明,去掉了也不会影响主句的意思,它与主句之间通常用逗号分开.
引导定语从句的关系代词有：that（指人或物）,which（指物）,who (指人）.指人时可用that或who ；指物时可用that 或which ；但有时只能用that,不能用which.常见的情况有下列五种：
1） 当先行词是：all , any , anything , everything , nothing或被它们修饰时.例如：
Is there anything that I can do for you ?
有什么可以为你做的吗?
All that can be done must be done .
凡是能做的都必须做.
2）当先行词被形容词的最高级或序数词修饰时.例如：
That is one of the most interesting books that are sold in the bookshop .
那是这家书店出售的最有趣的书之一.
The first thing that we should do is to get some food .
我们该做的第一件事是弄点吃的.
3）当先行词有：the only , the very , the same , the last 等修饰时.例如：
My necklace is not the only thing that's missing .
我的项链不仅是丢掉的东西.
4）当主句以who或which开头时,定语从句中引导词用that ,不用which 或 who .以避免重复.例如：
Who is the girl that is wearing a red coat ?
正穿一件红色外套的女孩子是谁?
5）当先行词既有人又有物时,用that .例如：
The writer and his novel that you have just talked about are really well known .
你刚才谈起的那位作家以及他的小说确实很著名.
