English 'anaphoric' words - words which 'stand for' other constituents of a sentence - are all called pronouns, but in fact there are two different categories:
- words which act as placeholders for nominal expressions (he, she, it, what, which, who, this, that &c)
- words which act as placeholders for adverbial expressions, and might better be called pro-adverbs (how, where, when, why, whither, &c)
But for adjectival expressions we have only the demonstrative such (now little used in this sense;sense) and we have nothing for verbal expressions. Instead we use the pro-nouns and pro-adverbs in various combinations.
- With adjectives we mostly use how+ADJ or what/which+NOUN as interrogatives/relatives and this/that/so/yay+ADJ or this/that+NOUN as demonstratives.
How big you want that? ... About yay big.
What color shirt was he wearing? ... That color.
- With verbs we mostly use what/which (interrogative/relative) and this/that (demonstrative), in combination with the all-purpose verb DO.
Whatcha doin? ... Whatchu see.
What do we do? ... What we have to do.
You could make a case for thinking of DO here as a pro-verb and what as its obligatory pronominal object.
