想請教有關in which的問題

in which基本上係formal句式寫作中常用作為引伸詳盡解釋時的連接詞
同理，你話在美劇中聽到用這作對話，我估計那對話亦是在解釋某事件或情況吧。
因此用in which的答案問不是在於formal還是informal，而是「認真度」，當人要表達解釋詳盡或把某事情抽絲剝繭(或以類比)地解釋時，便會用in which。不是認真地講解或論述的話，一般不會用in which來把一個簡單的回應複雜化地延長吧 - 例如: I think abc is OK. 而不會特意說: I think abc is OK, in which abc has the money, resources, knowledge, skills, wisdom, vision, influence, power, planning.... to complete the job. 除非他要說服他人而說明理據。

Which 嚴格來說, 不是連接詞 (conjunction), 是關係代詞 (relative pronoun) 或 限定詞 (determiner)

作為關係代詞, 它有雙重的功能,
相似其他的代詞 (pronoun)一樣表示已提及的事物, 同時地像連接詞, 因為 which 將主句 (main clause) 和從句 (subordinate clause) 連接起來. 另一個角度看, which 是將從句和所修飾的名詞 (noun) 或代詞連接起來. Which 總是從句之首.有時有前置詞.

The situation which he found himself in was very difficult.
The situation in which he found himself was very difficult. (formal)

將 in (前置詞) 放置在 which 前面, in which 是 formal 些, 多數是 written English或 academic English. 在正常會話中, 有人講, 特別在 public speech.
在第二句使用 in which, 不是解釋詳盡. 上述兩句意思一樣.

Politics is a topic which I have no interest in.
Politics is a topic in which I have no interest.

This is the house which I was born in.
This is the house in which I was born.

The rate which the material heats up at depends on its chemical composition.
The rate at which the material heats up depends on its chemical composition. (better, separating two prepositions)

其他的前置詞 + which, on which, to which, from which, for which
The ladder which I was standing on began to slip.
The ladder on which I was standing began to slip.

The club which he belonged to had just become too expensive.
The club to which he belonged had just become too expensive.

The death of her son was an experience which she never fully recovered from.
The death of her son was an experience from which she never fully recovered.

It was a disaster which the manager was unprepared for.
It was a disaster for which the manager was unprepared.

小心, 如果 relative clause 動詞是 2-word or 3-word phrasal verb, 前置詞不能放置在 which 前面. 例如 look forward to
The party, which I have been looking forward to all week, is at my sister’s house.

“in which” is used to refer something. “in which” 是提及的事物 (film)
Is it the film in which he robs the bank?

Which 作 限定詞 (determiner)

She may be late, in which case we should wait for her.
In which direction he is going?
She lost her temper, at which point I decided to go home.