I still do not understand why the sentence A has but sentence B does not?
A) He was in a quandary about which selection from his extensive repertoire would be feasible to perform for the children.
B) Home owners are not only tasked to choose which part of their house it should be their main priority but also....
Can someone give me a detailed explanation?
Well, we can speculate.
In each case, the main clause requires a complement, and the complement is extracted from a subordinate clause.
In (A), the subordinate clause is an infinitive. By default (i.e., as an independent clause), the infinitive looks like this:
To perform [something] for the children would be unfeasible
where [something] is expressed by "which selection from his extensive repertoire."
But since English isn't particularly fond of infinitives as subjects, the infinitive is shifted to the back, and "dummy it" appears at the front as the new (syntactic) subject. This "transformation" process is known as
extraposition.
It would be unfeasible to perform which selection from his extensive repertoire for the children.
And it is from this version that "which selection from his extensive repertoire" is extracted to fulfill the transitivity of the preposition "about" in the main clause:
He is in a quandary about which selection from his extensive repertoire
Next, the writer adds what's left of the subordinate clause:
He is in a quandary about which selection from his extensive repertoire it would be feasible to perform for the children
And that's how "it" appears in (A). (Writers do this intuitively.)
But there is no need to add dummy "it." The phrase extracted from the subordinate clause is a
noun phrase, and as noun phrase, "which selection from his extensive repertoire" can properly function as subject of the verb phrase "would be feasible:"
He is in a quandary about which selection from his extensive repertoire would be feasible to perform for the children.
In (B), there is no to-infinitive, no extraposition, and no "dummy it." In (B), there is an underlying clause.
their main priority should be [unknown]
where "which part of the house" represents the unknown.
From this, the noun phrase "which part of their house" (the unknown) is extracted to function as complement of "to choose" in the main clause:
Home owners are not only tasked to choose which part of their house
Then, the writer treats "should be" as a
single auxiliary, and does auxiliary-subject inversion (typical of direct questions) to underscore the "unknown:"
Home owners are not only tasked to choose which part of their house should be their main priority ...
And since there is no "it" in the underlying clause, there is no "it" in (B).
