Page 514 - Practical English Usage 3ed - Michael Swan, Oxford

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what not used
What is only used to mean 'the thing(s) which'. It cannot be used as an ordinary relative pronoun after a noun or pronoun.
We haven't got everything that you ordered. (NOT ••• el:letythillg whtJt ...)
The only thing that keeps me awake is coffee. (NOT The 671;Iy thing IIfflfJt . ..J We use which, not what, to refer to a whole clause that comes before
(see 494.9).
SaUy married Joe, which made Paul very unhappy. (NOT ••• whtJt ntNie ...)
what as a determiner
What can also be used as a determiner with a noun.
What money he has comes from his family. (= The money that he has ...) I'll give you what help I can. (= ... any help that I can.)
relatives (5): advanced points double use of relative pronouns
Note that relative pronouns have a double use: they act as subjects or objects inside relative clauses, and at the same time they connect relative clauses to nouns or pronouns in other clauses - rather like conjunctions.
relative pronouns as general-purpose connectors
In non-identifying clauses, the pronouns who and which sometimes act as general-purpose connecting words, rather like and + pronoun.
She passed the letter to Moriarty, who passed it on to me. (= •.• and he passed it on ...)
I dropped the saucepan, which knocked over the eggs, which went all over the floor. (= .•• and it knocked ... and they went ...)
I do a lot ofwalking, which keeps mefit. (= ... and this keeps me fit.) who and whom
Who can be used as an object in identifying clauses in an informal style. Whom is more formal.
The woman who I marry will have a good sense ofhumour. (More formal: The woman whom I marry ...J
In non-identifying clauses, who is less common as an object, though it is sometimes used in an informal style.
In that year he met Rachel, whom he was later to marry. (OR ••• Rachel, who he was later to marry. - informal)
that for people
That is often used in identifying relative clauses instead of whol whoml which (see 494.4). That is most common as an object, or as a subject instead of which. That can be used as a subject instead of who, but this is quite informal. Compare:
the people that I invited (normal) the booles that I lent you (normal) the bus that crashed (normal)
the people that live next door (informal; the people who . .. is preferred in a
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less informal style)
relatives (5): advanced points 498
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