1. which 用法
which 在英文文法内属于关系代名词(relative pronoun)， 后面接的句子是补充前句的额外资讯，指称「一件事/物件」，例子有﹕
I don’t know which pen is yours (我不知道那支笔才是你的)
在这种情况下，which 指的是那枝属于你的笔。
which亦可以用作连接两句句子，从而对该「事/物件」作出更多描述，例如﹕
Apple was founded by Steve Jobs (Apple由Steve Jobs 创办)
+
Apple has changed the world. (Apple改变了这个世界)
=
Apple, which was founded by Steve Jobs, has changed the world. (由steve jobs 创办的Apple改变了这个世界)
要记得，which 一定是跟在noun后面！
知道了which的基本用法和意思之后，我们就可以看看which 前面加上不同介词到底是什么意思啦! 首先是 in which﹕
2. in which 用法及中文意思
1. in whcih 可以理解为将句子后面的介词(preposition，就是in/ of/ on 等)搬到前面，从而令句子变得更加正式，例子有﹕
I grew up in this neighborhood.
+
This neighborhood has been rebuilt.
=
This is the neighborhood in which I grew up has been rebuilt.= This is the neighborhood which I grew up in has been rebuilt (我从小住的那个社区现在已被重建啦)
2. 形容「地点」
与此同时，当which是用来形容事物时，in which 就是用来形容「地点」，将两句句子连在一起， 同时亦可以用where代替，但in which 就有比较强调「在..里面」的意思，例如﹕
I once studied in this university
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this university is the best one in HK.
=
This is the university where I once studied is the best one in HK= This is the university in which I once studied is the best one in HK (强调他曾经在这家大学里面读过)
记得，如果想用where取代in which的话，需要连preposition(也就是in)都要拿走喔!
3. of which 用法
1. 介词前置
of which的用法和原理跟in which相似，首先同样是介词前置的用法﹕
Jane is now working in a company.
+
You must have heard of this company.
=
Jane is now working in a company which you must have heard of. = Jane is now working in a company of which you must have heard. (Jane现在的公司你一定都听过)
2. of which 的另外一个意思是「其中」。
当of which是用作连接两句句子的时候，of which 后面的那句就是在形容前句的其中一项物件，例如﹕
前句﹕There are 3 pens on the desk
+
后句﹕The pen in red is mine
=
There are 3 pens on the desk, of which the pen in red is mine.（桌上有3枝笔，其中红色那枝是我的笔）
在这个例子里面，of which 后面的 ’the pen in red’ 就是在说桌上3枝笔的其中一枝
4. by which 用法
介词前置
同样是介词前置的原理，请看以下例子﹕
He was known by a nickname at school.
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His nickname is ‘Spiderman’
=
The name by which he was known at school is Spiderman.（他以「蜘蛛人」的绰号在学校内非常有名）
2. by which 的另外一个意思是「藉由…」，
当by which用以连接两句句子时，前句就是后句「动作」的结果，例如﹕
前句﹕He learns English in many ways.
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后句﹕He takes online English courses to practice his oral English.
=
One of the ways by which he learns English is taking online English courses to practice his oral English. （他其中一个学英文的方法是藉由线上英语课程练习英语会话）
5. at which, on which, for which 用法及意思
和in which/ of which一样，基本上所有「介词+which」的第一个用法都是介词前置，也就是将后句最后的介词放在which 前面。
而意思上就每个字都有不同的意思，用法亦大同小异，同样是用后句形容前句，其中包括﹕
at which = 「在…某一点上」
The picture at which everybody is starring was painted by Da Vinci. (那幅人人都在望的画是达文西的作品)
for which = 「为了….」
I will buy you the reference book for which you asked (我会买你提过的那本参考书给你)
on which =「在..上面」
You can take my key from my desk on which there are many photos.(你可以在我的桌上拿我的钥匙，我的桌上有很多照片)
6. who/ whom用法
和which类似慨的relative pronoun还有who/whom, 不过形容的是人而不是事物。当你指的人是句子的subject时用who指称，如果是object就用whom指称，例如﹕
He is a teacher
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He teaches English online
=
He is a teacher who teaches English online.
在这句里面，’He’是主语，所以用who。又例如﹕
He loves the teacher very much
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The teacher teaches English on AmazingTalker
=
He loves the teacher whom teaches English on AmazingTalker
