Rule: Nominal relative clauses cannot be extraposed.
Examples of errors:
(1) It should be even more strictly observed what industries do with their waste.
(2) It can't be imagined by a European how black Americans feel about their white counterparts.
(3) It's interesting how they build up a star in show business.
Extraposition is the process by which a nominal clause (a clause which functions as a noun), usually the logical subject of the sentence, is moved from-its normal front position to the end of the sentence. The nominal clause which is moved in this way can be a finite, -ing participle, or infinitive clause, and it replaces the extraposed clause as grammatical subject:
That I can relax
Being able to relax
To be able to relax
is great.
It's great
that I can relax.
being able to relax.
to be able to relax.
A nominal relative clause is a nominal clause introduced by a wh- word which functions as subject or object in the matrix sentence (cf. 1.10.6). Nominal relative clauses are similar to interrogative clauses (indirect questions), but different in that most nominal relative constructions can be paraphrased by a noun phrase plus relative clause (hence the term nominal relative clause), e.g.:
I know
what you want
that which you want
Whoever
Anyone who
says that is a liar.
This is not possible with interrogative clauses:
I asked him
what he wanted
*that which he wanted
Furthermore, whether, if and who are used in interrogative, but not nominal relative, clauses.
In most cases nominal relative clauses cannot be extraposed. This would explain the error in (1), but there are cases for which this explanation does not suffice, for example:
It doesn't matter what you say.
It's not important how it happened.
These sentences contain extraposed nominal relative clauses and are to be distinguished from cases where the clause is not extraposed, but represents simply a repetition of the subject or some other element of the clause in expanded form--a kind of false start common in ordinary spoken discourse:
It's not important, what you said.
It's really interesting, this book.
I don't like him, that fathead.
Although there are, then, cases where nominal relative clauses can be extraposed, these cases seem to be limited to a very few verbs (matter, be important), and even then not in every instance; in the affirmative, for example, extraposition is more difficult:
*It matters what you say.
*It's important how it happened.