Extraposition is an expletive construction

Pronoun it is the subject

According to John Lawler, extraposition “seems to work to keep subject complement clauses from assuming the normal subject position preceding the verb.”

Besides applying to infinitive clauses, this grammatical use of it to delay a subject also applies to finite complement clauses, of which there are two types: that-clauses and embedded questions. (The non-finite complement clauses are infinitive clauses and gerund clauses.)

Read this article by linguist and general practitioner of linguistics from the University of Michigan John Lawler. It talks about the following two important syntactic rules that relate sentence structures:

1. Equi (short for Equivalent Noun Phrase Deletion)
2. Raising (short for Subject Raising)

Let’s have a look at the following sentence. It contains a complement (headed by the wh-word how) that functions as the subject of the sentence, but that subject (it) is delayed:

• It’s my prerogative that how I handle my own debt. (My prerogative is how I handle my own debt.)

As a rule, when a wh-interrogative (i.e., a nominal word such as what, where, when, etc.) heads a complement clause (i.e., subject or object clause), that clause has a function within the sentence. It is also important to note that the nominal word also has a function within the clause itself, but in this analysis, we will be focusing on the function of these complements within sentences. A grammatical function, as you know, relates what a word/nominal or a complement clause does in the syntactic arrangement (colour-coded). For example:

1. He held her, and with a single arm snaked across her back, he let her breathlessness turn into whatever it would. (The complement is the object of the preposition.)
2. They can give you way more than what this room with me and a bed can give you. (Than is functioning as a preposition, so the complement is an object of this.)
3. She told me how proud of me she was. (The complement is the direct object of the verb. Also note that me is the indirect object of the verb.)

Conversely, take a look at the following sentences:

1. It could not have been a lack of physical strength that she died.
2. Maybe it wasn’t such a terrible thing that he did.

In comparison to the previous three sentences, these complements serve no grammatical function within their respective sentences. Rather, the grammatical function of object is fulfilled by a noun phrase (bolded) and the complement is an appositive (which can be either a noun or a noun phrase) of the clausal object (the noun phrase). Notice that an appositive renames the noun phrase that came right before it.

It is now important to state that neither of these last two examples could use a wh-pronoun. Instead, an expletive (i.e., that, if, whether) must be used. This is because wh-pronouns, by their definition as complementizers, turn a clause into a subject or object. Therefore, these sentences feature their complement clauses not as a grammatical subject or object but as an appositive, which has no grammatical function.

But in our original example the complementizer indeed has a function (that of grammatical subject); however, the structure of the sentence may stymie your parsing to identify the true grammatical subject. For at first glance, it appears that the main clause (it’s my prerogative) is replete with subject (it), verb (is), and object (my prerogative). So you might expect that an expletive, not a nominal or interrogative word, as in the case of how, is called for. But now recall the rule that when such a word heads (or is the object of) a complement clause that word has a function within the sentence. So the use of an expletive here would be inaccurate.

Let’s go back to the beginning of this analysis, where I provided a link to the two important syntactic rules that relate to sentence structures. I would also like to mention now that extraposition is also a syntactic rule, similar to Equi and Raising. Extraposition is what has been applied to the “my prerogative” example to the effect that the main clause has the pronoun it as its subject. However, this pronoun has merely taken the place of the true subject, which is in the form of a complement clause. Let’s look at the proposition again:

• It‘s my prerogative how I handle my own debt.

This indefinite use of the pronoun it is used situationally (to delay the subject, i.e., “Who is it?” “It’s Garie,” rather than just “Garie”), which is not covered under the scope of this analysis. This scope covers when it is used expletively, which is when a complement acts as a subject and the indefinite pronoun substitutes for that subject, as illustrated just above. Whether used situationally (wh-cleft sentences) or expletively (it-cleft sentences), this indefinite use of the pronoun it invariably gives the sentence emphasis.

It is the subject of the predicate is my prerogative, but the interesting thing is that for the pronoun to assume a position that does not rightfully belong to itself results in what is called, again according to John Lawler, a dummy it subject. Therefore, the rightful subject is the complement, how I handle my debt.

That fact is now indisputable when the sentence is restructured as follows:

• How I handle my debt is my prerogative.

This construction is as a result of the Tough-Movement, which, according to Lawler, “raises the object of the complement clause to become subject of the main clause.” (How is the object of I handle my debt.)

Similarly:

• It was Christine who did something.

BECOMES

• Christine did something.

In an inflected language like Spanish, there’s sufficient freedom in word order to allow subjects to be placed just about anywhere on a stylistic whim; but English is rather fussier about where things go, and it just won’t do to have subjects lying about anywhere at all. You have to have something up there at the beginning, and it is the traditional choice.

Anticipatory “it” can also be a subject, John Lawler

SUMMARY/OBSERVATIONS: By applying the Tough-Movement, you can see that a relativizer, not an expletive, is called for when the indefinite pronoun it is used expletively (clefting).

I happen to agree with professor Lawler that English is rather fussy.