in which,which

which 係個關係代名詞 (relative pronoun)，在關係從句中可作主詞或作受詞。

以下兩句可以用關係從句合併為一句。
My father gave me a new camera. The camera has a lot of cool features.

which 作主詞例句：
My father gave me a new camera which has a lot of cool features.

which 作受詞例句：
The camera which my father gave me has a lot of cool features.

which 在關係從句中給要說明的物件提供額外資料。


in which 是連接詞，是 where 的意思。
I live in a city which is densely populated. (which 是代名詞，作主詞)
The city which I live in is densely populated. (which 是代名詞)
這句可寫成
The city in which I live is densely populated. (in which 是連接詞)
The city where I live is densely populated.

Another example:
Next to the house is a pond. You will see many fish in the pond.
Next to the house is a pond in which you will see many fish. (in which 是連接詞)
Next to the house is a pond where you will see many fish.

used to be exact about the thing that you mean has 'which"
eg Houses which overlook the Victoria Harbour cost more.
used to give more information about something has"in which"
eg His best movie in which won several awards was called "Amah sister Ah Tao".
Your claim in which Millions be the sum,ought to succeed.
"That" can be used instead of in which in meaning
eg It was a rape that she was unprepared for.
=It was a rape in which she was unprepared.