I don't think that it is all that clear-cut. A what clause is sometimes regarded as singular and sometimes as plural.
When what is the object it is usually followed by plural agreement, as in What we need in government .. are men who seek to understand issues in all their complexity.
When it is the subject, we more often find singular agreement, as in What we are getting is old answers to old questions. [...]
As Welkin already spotted, there's no difference in construction between those two sentences.
"What" is the object of the
what-clause in both cases, so if
"is old answers" is correct (and I maintain that it is) I don't see an argument there for
"are men".
In
"What is the only problem [are] is the bubbles", "what" is the subject of the
what-clause, and is clearly singular (because we see
"What is"). But what governs whether the main verb should be
"is" or
"are" is whether its subject (the whole
what-clause) is singular or plural. In this construction,
"what" is sometimes called a fused relative pronoun, because it stands for
"that which". So the
what-clause can be rewritten as
"That which is the only problem".
That gives us a true relative clause (
"which is the only problem") modifying
"That".
"That" is the head word of the subject of the main clause, and it's singular, so the main verb must be singular:
- "That [which is the only problem] is the bubbles" (not
"That [which is the only problem] are the bubbles"!). Similarly ...
-
"What is the only problem is the bubbles" (not
"What is the only problem are the bubbles"!)
The fact that Messrs Turpin and Truman have written obviously singular
what-clauses followed by
"are" simply shows that not everyone writes grammatically. How much their readers care is of course another question — but what those gentlemen are doing is making the main verb agree in number with the subject complement and not with the subject.
"What impresses them are planes ..." is the same construction as
"The subject of the book are planes ...", yet I can't imagine they'd ever have written the latter.
Is it just me or do your first two sentences both have 'What' as the object in the clause and the subject in the whole sentence?
"What" is indeed the object in each of the
what-clauses, but it's not the subject of the whole sentence. The subject of the main clause is the whole
what-clause.
Ws