Translation:He arrived thirty seconds earlier than I.
I just submitted two reports for this. I got confused because I think maybe they half corrected it? I wrote "He arrived thirty seconds earlier than I," and the answer was given as exactly that. Nevertheless, I got "almost right!" and the pop-up message that "I needs to be me."
So it looks like the answer is now correct (no idea if it always was or not), but the pop-up message is incorrect. (As Jantoki is no doubt aware, "He arrived earlier than me" is very commonly used these days, but is still considered technically incorrect according to grammar as taught in school.)
Sorry to the course moderators for the double report! It took me a minute to realize what was going on.
Big debate "Than" conjunction or preposition?
Outlined here and here:
"She likes him more than I (like him)" = we both like him but she does more.
"She likes him more than (she likes) me" = she prefers him to me.
Allowing 'than me' for 'than I' would lead to ambiguities in sentences like the one above, when keeping it separate like this would avoid it.
As previous comments have mentioned, "than me" is by far more common in modern English, and "than I" sounds very odd/formal/outdated to most English speakers. English is not a prescriptive language, what's correct is determined by the consensus of its speakers, and therefore "me" is correct unless it's a situation that requires an unusual degree of formality.
People might not, but I would. However, I would always add the verb "did" because that poor old subject pronoun on its own invariably invites argument.
English plays fast and loose with whether pronouns should be accusative or not. For instance, "I did" is a lot less common than "me" to the question "Who did this?" Or "Who's there?" then "It's me: Jimmy."
The rules, if they're to be right or meaningful, have to reflect the fact that those examples are normal English.