I’m confused when to use “in which” and “which”. Can someone explain it to me?
And please let me know if there’s anything grammatically wrong in my sentences.

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@wonys555

Study the following examples and you will understand when.
Example 1:

The room where I sleep has two windows.

This is the same as : The room which I sleep in has two windows.

and the same as: The room in which I sleep has two windows.
Sometimes people want to avoid ending a sentence with a preposition, so instead of saying “This is the house my mother lives in,” they will say “This is the house in which my mother lives.” You can also say “This is the house where my mother lives.”
In formal writing, “in which” might be used more often, but in casual speech, it sounds stuffy and overly formal.
Remember you can also use “which” as a question word, so you might find “in which” used this way: In which hotel will you be staying? Here, there’s a choice of hotels, and I’m asking you to identify the one where you plan to stay.
If your sentence has a clause but does not need it, use "which" .
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