Double subject construction?

Question

I've recently been having trouble with constructions that seem to mark two subjects in the same clause. I've only encountered them in ～の方が... constructions, but I can't seem to make sense of them grammatically. Here are some examples:

1. 当然、都心より郊外のほうが家賃が安い。 (from a vocabulary book)
2. （人）よりも自分の方が知識があると思う (from アルク)

My questions are: why are two subjects allowed here, and are there other constructions in which this happens? So far my thoughts on this are:

(a) 方 seems to be a subject, but not have a verb. So maybe this is simply be some kind of relative-clause-like construction with the noun elided away, e.g. 家賃が安い[ところだ] and 知識がある[人だ]. But this seems a little odd - I don't know of anywhere else where this is possible.

(b) The second sentence might bracket as （人）よりも自分の方が([blah]と思う), rather than (（人）よりも自分の方が[blah])と思う. This makes perfect sense, but I suspect that the と思う could be removed to leave a valid sentence, so this explanation seems a little spurious. In any case this doesn't explain the first sentence.

So I'm a little confused. Any light anyone can shed on this would be great. Thanks!

Answer 1

As @Flaw flawlessly explains, Japanese sentences can have clausal predicates. This is what causes what is commonly known as double-subject constructions, although I believe "clausal predicates" really illustrates the structure better.

I assume you have heard constructions like

彼は髪が長い He has long hair

Some teachers/textbooks might explain this away by saying that 彼 is a topic, not a subject, but that creates problems when you run into sentences like

彼が髪が長いんだ！ He is the one with long hair!

Here the first が is an exhaustive-listing が. When multiple がs occur in a main clause, the first is usually exhaustive-listing. Note that 髪が長いのは彼だ might be more common, but I don't consider the above ungrammatical. Also, dependent/relative clauses cannot have topics, so you might see

彼が髪が長い理由は ... だ The reason for his long hair is ...

although I think in this case 彼の髪が長い理由は...だ might also be as/more common.

In your example sentence, のほう needs to have a が (This would also fall under the exhaustive-listing category) to have the comparative meaning

郊外のほうが家賃が安い Rent is cheaper in the suburbs

Changing the が to a は would lose the comparative meaning

郊外のほうは家賃が安い Rent is cheap in the suburbs

As a side note, I'm not exactly sure what the function of のほう would be in this case, I think you can see it either as a filler, or as something that directs your attention to 郊外. 郊外は家賃が安い would mean almost the same.

To address Billy's question in his comment

郊外が家賃が安い It is in the suburbs that rent is cheap

is grammatical, but quite narrow in meaning. And again, 家賃が安いのは郊外だ is probably more common to convey this meaning.

Answer 2

I think this is how:

Consider first Clause₁: 家賃が安い.
The structure is Subject₁+が+Predicate₁.
Subject₁: 家賃
Predicate₁: 安い

Now consider Clause₂: (都心より)郊外のほうが家賃が安い.
The structure is Subject₂+が+Predicate₂, where Predicate₂ is Clause₁.
Subject₂: 郊外のほう
Predicate₂: 家賃が安い

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Predicate: The part of a sentence or clause saying something about the subject