Hi eipjoo This is a difficult question even for native speakers. I found a very useful and interesting blog on this - see this link. html Quoting from this blog: " In a nominal relative clause, the nominal relative pronoun refers to something known, something concrete, which can be specified.
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eipjooI can’t even remember what I'm famous for.I'd call it a fused relative construction. 'what' ~ 'that which' ~ 'the thing that'