What will native speaker think about if I use such sentence as:
The man from whom I borrowed the book.
The library at which we met.
The day on which we met.
The reason for which I was at the library.
I think they're too formal and stilted for everyday use.
None of those sentences would sound natural in spoken English, other than in very formal contexts, such as a courtroom. In fact, the last one would never be used - it should be "The reason why I was at the library."
Here's how we'd say them in normal conversation:
The man I borrowed the book from.
The library where we met.
The day we met.
The reason I was at the library.
See? We don't actually need relative pronouns at all in three of the four sentences. You could use relative pronouns, but it's easier - and perfectly acceptable - to miss them out.
I hope that helps.
They're right but instead you can use who, where, when and why.. these are much common
But i guess both are right
Yeah, reading the sentences again the other two posters are correct. The examples Su. Ki provided are much more natural and commonplace.