What as a Relative Pronoun
Level C1 · Relative Clauses
what used as a relative pronoun means 'the thing(s) that' and introduces a nominal (noun) relative clause — a clause that itself acts as a subject, object, or complement in the sentence, unlike normal relative clauses (who/which/that) which only add extra information about a noun already mentioned. What I need is more time means 'the thing that I need is more time' — the whole clause what I need functions as the subject of the sentence.
How what works as a relative pronoun:
- what + clause functions as a noun phrase, standing in for 'the thing(s) that': I don't understand what you mean (object of understand); What surprised me was his silence (subject of the sentence); This isn't what I ordered (complement after is).
- Unlike who/which/that, what does not refer back to a noun already mentioned — it already contains its own meaning ('the thing that'), so there's nothing before it to point to: compare The book that I read (that refers to book) with What I read was fascinating (what needs no earlier noun).
- what + clause is common at the start of a sentence for emphasis, fronting the important information: What I really want is a coffee (emphasizes 'a coffee' by pushing it to the end).
- Don't confuse this what with the question word what in indirect questions — I don't know what she wants can be either an indirect question or a nominal relative clause, and both are grammatically identical in form, though a true relative use always means 'the thing(s) that'.
The most common mistake is using what after a noun that's already been mentioned, where a normal relative pronoun (that/which) is needed — the book what I read is wrong in standard English; it must be the book that/which I read, because book is already named and only needs a normal relative clause, not 'the thing that'. A second mistake is adding an extra 'that' — what that I need is wrong; what alone already means 'the thing that', so don't double it up.
Use what only when there's no noun before it for the clause to attach to — the whole what-clause acts as its own subject, object, or complement, and can usually be replaced by 'the thing(s) that' without changing the meaning: What you said (= the thing that you said) upset me. If there's already a specific noun just before the gap (the book, the person, the place), use that/who/which instead — never what.