If English words were Norse gods, perhaps than would be the best candidate to play the role of Loki, the trickster god.* It causes confusion not only due to its similarity with then, but also by raising the question of what case the noun phrase it governs should have. Of course, there’s no confusion if you are a brilliant grammaticaster, as in this example from a list of peeves:
12. I/Me: We had several different takes on this, with one correspondent nailing it thus: “The correct choice can be seen when you finish the truncated sentence: He’s bigger than I am. ‘He’s bigger than me am’ actually sounds ridiculous and obviates the mistake.”**
Now, there’re two questions one should be asking of this explanation. First, can you just fill in the blank? By which I mean, does a sentence with ellipsis (the omission of words that are normally syntactically necessary but understood by context) necessarily have the same structure as a non-elided sentence? There are many different types of ellipsis, so this is a more complex question than I want to get into right now, but the short answer is no, and here’s a question-and-answer example:
Who drank my secret stash of ginger-grapefruit soda?
(1a) He/*Him drank it.
(The asterisks indicate ungrammatical forms.) The second question to ask here is whether there even is an elision. We know that “He eats faster than I do” is a valid sentence, but does than necessarily trigger a clause after it? Could it be that “He eats faster than me” is not an incomplete version of the above structure but rather a different and complete structure?
This boils down to the question of whether than is strictly a conjunction (in which cases the conjoined things should be equivalent, i.e., both clauses) or can function as a preposition as well (in which case the following element is just a noun phrase, with accusative case from the preposition). It’s pretty easy to see that than behaves prepositionally in some circumstances, pointed out in the Cambridge Grammar of the English Language, via Language Log:
(2a) He’s inviting more people than just us.
(2b) I saw no one other than Bob.
But this prepositional usage has become frowned upon, save for instances like (2a) & (2b) where it’s unavoidable. Why? Well, it’s an interesting tale involving the love of Latin and the discoverer of dephlogisticated air. It all starts (according to the MWDEU) with Robert Lowth, who in 1762 claimed, under the influence of Latin, that than is a conjunction and noun phrases following it carry an understood verb. The case marking (e.g., I vs. me) would then be assigned based on the noun phrase’s role in the implied clause. So Lowth’s grammar allows (3a) and (3b) but blocks (3c):
(3a) He laughed much louder than I (did).
(3b) He hit you harder than (he hit) me.
(3c) *He laughed much louder than me.
Lowth’s explanation was not without its wrinkles; he accepted than whom as standard (possibly due to Milton’s use of it in Paradise Lost), and worked out a post hoc explanation for why this prepositional usage was acceptable and others like (3c) weren’t.
Lowth’s explanation was also not without its dissenters. Joseph Priestley, a discoverer of oxygen, argued against it in his Rudiments of English Grammar (1772 edition). Priestley noted that the combination of a comparative adjective and than behaved prepositionally, and that good writers used it as such. Regarding the Lowthian argument against it, he wrote:
“It appears to me, that the chief objection our grammarians have to both these forms, is that they are not agreeable to the idiom of the Latin tongue, which is certainly an argument of little weight, as that language is fundamentally different from ours”
Another pro-prepositionist was William Ward, whose 1765 Essay on Grammar argues that than in phrases like to stand higher than or to stand lower than is akin to prepositions like above or below, and thus that the prepositional usage of than should be allowed. That’s not much of an argument, because semantically equivalent words and phrases don’t have to have the same syntax — consider I gave him it vs. *I donated him it. But then again, Lowth is basing his argument on a completely separate language, so this is a slight improvement.
The real key, of course, is usage, and the MWDEU notes that Priestey and Ward are backed by standard usage at the time. Visser’s Historical English Syntax strengths the case with examples of prepositional than from a variety of estimable sources, including the Geneva Bible (1560), Shakespeare (1601), Samuel Johnson (1751, 1759), and Byron (1804). And of course prepositional than continues in modern usage — why else would we be having this argument?
Despite the irrelevance of his argument, Lowth’s opinion has stuck through to the present day, reinvigorated by new voices repeating the same old line, unwilling to concede something’s right just because it’s never been wrong. It’s the MWDEU, not the commenter mentioned at the top, who nails it:
“Ward’s explanation [that both conjunctive and prepositional uses are correct] covered actual usage perfectly, but it was probably too common-sensical—not sufficiently absolutist—to prevail.”
So in the end, we’re left with this. Than I and than me are both correct, in most cases. Than I is often regarded as more formal, but interestingly it’s the only one that can be clearly inappropriate.*** Using the nominative case blocks the noun phrase from being the object of the verb. I can’t write The wind chilled him more than I to mean that the wind chilled me less than it chilled him.
Sometimes this can be used to disambiguate; I love you more than him is ambiguous while I love you more than he isn’t. This only matters for clauses with objects, and in general, these tend not to be ambiguous given context, so the benefit of disambiguation must be weighed against the potentially over-formal tone of the nominative case. (I, for instance, think the second sentence above sounds less convincing, even though it’s clearer, because who talks about love so stiltedly?)
I’m branching a little off topic here, but I’d like to conclude with a general point from Priestley, a few paragraphs after the quote above:
“In several cases, as in those above-mentioned, the principles of our language are vague, and unsettled. The custom of speaking draws one way, and an attention to arbitrary and artificial rules another. Which will prevail at last, it is impossible to say. It is not the authority of any one person, or of a few, be they ever so eminent, that can establish one form of speech in preference to another. Nothing but the general practice of good writers, and good speakers can do it.”
Summary: Than can work as a conjunction or a preposition, meaning that than I/he/she/they and than me/him/her/them are both correct in most situations. The latter version is attested from the 16th century to the present day, by good writers in formal and informal settings. The belief that it is unacceptable appears to be a holdover from Latin-based grammars of English.
*: Interestingly, and counter to the mythology I learned from the Jim Carrey movie The Mask, Wikipedia suggests that Loki may not be so easily summarized as the god of mischief.
**: I’m pretty sure the use of obviate is a mistake here. I can just barely get a reading where it isn’t, if thinking of the full version of the sentence causes you to avoid the supposed mistake. But I suspect instead that our correspondent believes obviate means “make obvious”, in which case, ha ha, Muprhy’s Law
***: Is there a case where than me is undeniably incorrect? I can’t think of one.