在定语从句中，出现频率最高、也通常被认为是最简单的引导词就是that。that引导定语从句时that本身作为关系代词，在句中可以指代人也可以指代物，在从句中充当主语、宾语或者表语。另外，值得注意的是that不能引导非限定性定语从句。
that引导定语从句并在从句中充当主语：
例句1：He is the right person that/who can bring you new thought and passion.
他正式那个而已给你带来新思想和激情的人。
注释：此时that和who可以互换，指代前面 出现的"人"，that在从句中定语从句中充当主语。
例句2：A prosperity that/which has never been seen appears in the countryside.
农村地区出现了前所未有的繁荣。
注释：此时that和which可以互换，指代前面出现的"繁荣"，that在从句中充当主语。
that 引导定语从句并在从句中充当宾语：
例句1：That is all the information that/which I know.
这是我所知道的所有信息。
注释：此时that和which可互换，指代前面的"信息"，that在从句中充当宾语。
例句2： The old man that/who we met in the street yesterday is my teacher.
昨天我们在街上遇见的那个老人是我的老师。
注释：此时that和who可以互换，指代前面的"老人"，that在从句中充当宾语。
that引导定语从句并在句中充当表语：
例句1：Our country is not the poor country that it used to be.
我们的祖国再也不是曾经那个平穷的祖国了。
注释：关系代词that在从句中充当表语，指代前面的"贫穷的国家"，要注意的是关系代词在句中充当表语时只能用that。
定语从句引导词只用that不用which的情况：
(1) 关系代词that指代"物"并在定语从句中充当表语
例句：Ours is not the poor city that it used to be.
我们的城市再也不是以前那个贫穷的城市。
(2) 定语从句出现在there be句型中修饰该结构中的实意主语
例句：Along the road of pursuing dreams, there is much hardship that we have to live with.
在追逐梦想的路上，我们要忍受很多的艰辛。
(3) 当先行词是不定代词anything, everything, nothing, the one, all, much, few, any, little时
例句：All that you need to do is to finish your assignment on time.
你所要做的事情就是要及时完成你的任务。
(4) 当先行词即有人又有物时
例句：The picture is always reminding me of the people and things that have left me deep impressions during my short stay in Shanghai.
那张照片总是让我想起我在上海那段短暂时光里给我留下了深刻印象的人和事物。
(5) 当主句是以who, which开头的特殊疑问句或先行词本身是who, which时
例句：Which is the book that I bought? 哪本是我买的书？
Who that you have ever seen can defeat him? 你所见过的哪个人可以打败他？
(6) 当先行词有 the only, the very, the same修饰时
例句：This is the only book that you need to read at the moment.
这是你眼下需要读的唯一的一本书。
(7) 当先行词被序数词或形容词最高级修饰；或者先行词本身是序数词或形容词最高级时
例句：You are the best student that I've ever taught. 你是我教过的最好的学生。
He is the first that will come for you. 他是第一个能来帮你的人。
通常用which而不用that的情况：
(1) 引导非限定性定语从句时
例句：The tree, which is four hundred years old, is the signal of this small village.
这颗四百多年的老树是这个小村庄的标志。
注释：这里是由which引导非限定性定语从句对前面的名词进行补充说明，此时不能用that。
(2) 引导定语从句的关系代词作介词宾语时
例句：Money is not the matter about which you need to worry.
钱不是你担心的事情。
注释：这个句子也可以写成"Money is not the matter which/that you need to worry about."此时用that或者which都可以。但是，如果介词提前就只能用which来充当介词about的宾语。
(3) 当先行词本身就是代词that时
例句：We appreciate little that which we have and long for that which we do not have.
我们往往都是不珍惜已经拥有的东西而追求自己没有的东西。
注释：此时先行词是代词"that"，为了避免混淆通常就只用which来引导这个定语从句。
