Derrick Hempel wrote:
Q: How many subjects are there in Tomatoes might have been a better buy than corn?
A: One or two, depending on how you look at it.
Obviously tomatoes is the subject of the main clause, but the corn can be seen as the subject of a reduced comparative clause, the full version being Tomatoes might have been a better buy than [the corn is ___]. Alternatively, we could see the corn as the object of the preposition than.
You might be wondering about the gap up there. Of course, *the corn is is not a complete clause. Is demands a complement, and in this case, it’s looking for a good buy. The standalone clause would be the corn is a good buy. But comparative clauses always have a gap representing, at least, the comparative governor, the syntactic constituent denoting the characteristic on which the two things are being compared.
This can be minimal as in: It’s as long as [it is __ tall]. Here the comparative clause is missing only the degree modifier. One possibility for the gap would be 2 meters: *It’s as long as [it is 2 meters tall], meaning “it’s 2m long and 2m tall”.
Often though, much more is missing, even the verb can be gapped out: She chose the tomatoes rather than [they __ her]. Or, as is the case with Derrick’s sentence everything except the subject may be missing.
There are, however, comparatives that cannot in any way be viewed as being reduced clauses:
-  i. The tomatoes will be there no later than 7:00.
- ii. Rather than corn, you should buy tomatoes.
To make it really clear consider the difference between:
-  i.The tomatoes are heavier than the corn ___.
- ii. The tomatoes are heavier than a kilogram.
Clearly, then, than can be followed by either a comparative clause or an object (NP), and in Derrick’s sentence’s case, there’s no good reason to favour one analysis over the other.
The reason Derrick asked this question was because the sentence appeared in an exercise asking students to underline the subject in each sentence. A student was perceptive enough to question whether the corn could also be a subject. This highlights the need to select sentences for exercises very carefully and with a full understanding of the relevant grammar.
Many traditional grammarians have asserted that than is a “conjunction” only, never a preposition. As such, the tale goes, it conjoins two clauses, even when the subsequent clause isn’t all there. Following this logic to the bitter end, any pronoun following than must be the subject of the second clause and so should be in the nominative (or “subject case”). For example, you should say We are younger than they, not We are younger than them.
The first, though, is stuffy enough to require decongestant, and a “conjunction” analysis cannot account for cases like those in  and [2b]. Most usage guides agree that than is both a conjunction and a preposition (though not at the same time.)
The analysis of Huddleston and Pullum’s Cambridge Grammar of the English Language agrees with this in principle, but simplifies things slightly by allowing for prepositions to take clausal complements. Under this analysis, than is a preposition in both cases, just one that, like before, after, and many other prepositions, takes either NP complements or clausal complements.
Grammarology 2.0 aims to take the discussion of grammar up a notch or two (your mileage may vary). It’s written by Brett Reynolds, who teaches EAP at Humber College and is just about as nerdy about grammar as you can get. He’s is always happy to take questions, which you can send to firstname.lastname@example.org.