what vs which

[rte.sushil]

i understand that which is a relative pronoun and what is not.
But i am not able to express in words when to use what in the sentence.

such as :-
A leading figure in the Scottish Enlightenment,Adam Smith wrote two major books that are to democratic capitalism what Marx’s Das Kapital is to socialism

here what is used to clarify for comparison.

are there some rules that can help to identify when can we use "what" in sentences in places other than interrogative pronoun (what happened/what do you know?)or an interrogative adjective (what things you don't know)?

[RonPurewal]

I don't know the grammar terms, so I'll demur from that part of the discussion.

This "what", though, is the same "what" that appears in sentences like Please tell me what you saw.

Here's how this sentence works, by analogy:

Lesley has two children.
Adam Smith wrote two books.
This should be straightforward.

Lesley has two children who are her world.
Adam Smith wrote two books that are classics.
Adam Smith wrote two books [b]that are what Das Kapital is to socialism.
Same grammar in all three. The third one is nonsense, because it only contains half of a comparison, but I just want to show you how it is built up. ("What xxxxx" takes on the role of a noun.)

Lesley has two children who are the world to her.
Adam Smith wrote two books that are classics in economics.
Adam Smith wrote two books that are to democratic capitalism what Das Kapital is to socialism.
Same structure again.

[RonPurewal]

Just to clarify"”
In the sentence here, the green part is moved in front of the pink thing, just because the pink thing is so long that the sentence would be unreadable without it.

The same transition is common in other instances, too:

I dedicated a song to my father
BUT
I dedicated to my father a song commemorating the times we had spent together when I was a small child
(It's possible to write "I dedicated to my father a song", too, but that would be weird unless you were writing poetry.)

For the record, I have no idea why the bold tag still appears in my post above. I checked it, and it's properly written and closed off.
Oh well.