Advanced relative clauses include structures with quantifiers (some of which, many of whom), prepositions (in which, for whom), and sentential relatives that comment on entire clauses. These structures are formal and commonly used in academic and professional writing.
Quick tip: Quantifier + of which/whom for numbers. Preposition before which/whom for formality. Sentential which (with comma) for whole clauses.
Quantifier + of + which/whom
Use quantifiers (all, some, many, most, none, both, each, neither, etc.) + of which (things) or of whom (people) to give information about a quantity from the group mentioned. Very formal.
She has three brothers, all of whom are doctors. (All three are doctors)
I visited five countries, most of which were in Asia. (Most of the five were in Asia)
They invited 20 guests, some of whom I knew. (I knew some of the 20)
We interviewed ten candidates, none of whom was suitable. (Not one of the ten was suitable)
Preposition + which/whom
In formal English, prepositions can come before the relative pronoun which (things) or whom (people). This is more formal than ending with a preposition (that/which...preposition).
The house in which I grew up was demolished. (= The house I grew up in)
The person to whom I spoke was very helpful. (= The person I spoke to)
The principle on which this is based is simple. (= The principle this is based on)
The manner in which he spoke was rude. (= The manner he spoke in / the way he spoke)
Sentential Relatives (which referring to a clause)
Use which (always with a comma) to refer to the entire previous clause, not just one noun. This adds a comment or consequence about the whole situation described.
He arrived late, which annoyed everyone. ('which' = the fact that he arrived late)
She passed the exam, which surprised her parents. ('which' = the fact that she passed)
They cancelled the meeting, which was a relief. ('which' = the fact that they cancelled it)
The project failed, which cost the company millions. ('which' = the fact that it failed)
Common mistakes
Not "She has two sons, both of them are engineers." — say "She has two sons, both of whom are engineers.". Use of whom (relative clause), not 'of them' (separate sentence). Don't make two separate sentences with a comma..
Not "The hotel in that we stayed was excellent." — say "The hotel in which we stayed was excellent. OR The hotel that we stayed in was excellent.". After a preposition, use which not 'that'. Or move the preposition to the end and use 'that'..
Not "He failed the test which made him very sad." — say "He failed the test, which made him very sad.". For sentential relatives (referring to the whole clause), you must use a comma before which..
Practise it
Formal style: preposition before which/whom. Informal: preposition at end. Always use comma with sentential which.