Advanced relative clauses go beyond basic who/which/that structures. They include free relatives (whatever, whoever), nominal relatives (what = the thing that), and complex prepositional structures common in academic and formal writing.
Quick tip: 'What' means 'the thing that'; 'whatever/whoever/whichever' mean 'any X that'; use 'whom' after prepositions.
Free relatives: whatever/whoever/whichever
Use whatever/whoever/whichever to mean 'anything that/anyone who/any one that'. They can be subject or object.
Whatever you decide, I'll support you. (= anything that)
Whoever calls should leave a message. (= anyone who)
Choose whichever option suits you. (= any option that)
Nominal relatives: what = the thing that
Use what to mean 'the thing(s) that'. It functions as subject or object of the sentence.
What concerns me is the budget. (= the thing that concerns me)
I'll do what I can. (= the thing that I can do)
What she said was surprising. (= the thing that she said)
Complex prepositional relatives
Use formal phrases like as a result of which, in the absence of which, the majority of whom in academic and legal writing.
The merger failed, as a result of which hundreds lost jobs.
The study examined 200 participants, the majority of whom were female.
He lost the key, in the absence of which we couldn't enter.
Common mistakes
Not "Whatever you will decide, I'll support you." — say "Whatever you decide, I'll support you.". Use present simple after whatever/whoever/whichever, not 'will'.
Not "The thing what concerns me is the budget." — say "What concerns me is the budget.". 'What' already means 'the thing that' - don't add 'the thing'.
Not "The majority of who were female..." — say "The majority of whom were female...". Use 'whom' (not 'who') after prepositions like 'of'.
Practise it
Practice replacing 'the thing that' with 'what' and 'anyone who' with 'whoever' to make sentences more natural and concise.