在定语从句里 in which ，for which或者under which有什么区别？怎么用这几个句子 谢谢

which 是定语从句中关系代词，类似的还有 who, whom, that, what等等，用于修饰主句中的先行词的。

in ，for，under 等都是介词， 用于当后面的定语从句解释修饰先行词时的介绍关系，具体就要看定语从句和先行词的关系来定了。 例如：It is the classroom in which I have studied for 3 years. classroom就是先行词，定语从句说的意思是在这间classroom里学习过三年，就必须在关系代词which前面加上in。

你好。in,for和under的使用由从句部分的谓语决定：
1 There are cases. In those cases she comes late.
IN ------ There are cases IN which she comes late

2 Tell me the reason. You came late FOR that reason
For ----- Tell me the reason FOR which you came late

3 There are circumstances in our life. The right thing to do may become the wrong thing to do UNDER those circumstances
UNDER -------There are circumstances in our life UNDER which the right thing to do may become the wrong one to do

4 祝你愉快。

例如，this is the town in which i was born=this is the town which i was born in
用拆句子的方法：可以拆成this is the town.I was born in the town.
即动词后没有介词，又不能拆成完整句子，就要在which前加借此
一般表示“在。。。”时，用in which
“为了。。。”时，用for which
“在。。。之下”时，用under which

那要看定语从句修饰的对象啊 ～

不同的动词会搭配不同的介词。