This page could be helpful for those of us confused about revenir/retourner/rendre: http://www.frenchtoday.com/blog/aller-venir-retourner-revenir-rentrer-to-go-back-to-come-back
"revenir" is intransitive, it means "to come back".
"retourner" is transitive or intransitive: "je retourne chez moi" (I go back home) or "je retourne ce livre à la bibliothèque" (I return this book to the library)
"rendre" is transitive: "je rends la monnaie" (I give back the change)
"se rendre" is pronominal and intransitive: "je me rends au marché" (I am going to the market).
Someone posted this article earlier http://french.about.com/library/weekly/aa072900.htm
it said that when money is involved you use "rembourser". Is this not the case in every day spoken french?
You probably know this by now, but somebody else might be asking themselves the same question.
Because "seconde, f" is the word for the unit of time. Also for the musical interval. And when shifting gears in a car.
"second, m" is the word for a specific position, as in "second in command", "second mate onboard a ship", "the second in a duel", "second floor".
Why are synonyms of "a second" (I tried "a moment") not accepted? I'm not a native speaker, but I would say "He returns in a minute." or "He returns in a moment.", not "He returns in a second" (can I ask some of you native English speakers which of these variants are normal and which are rare in English?).
Anyway, the meaning of all three sentences is the same (the words are different, but the differences are irrelevant in this context), so they should be accepted too.
We would say "is returning" rather than returns. But yeah all of those sentences have the same meaning. And they're all common so it doesn't really matter which one you use in speech. but, I probably wouldn't translate the sentence above into minute or moment because it is quite a direct translation from seconde to second.
Canadian and US English speakers wouldn't say either of those. We use these commonly:
He'll be back in a moment/second/minute, in a few (informal).
He'll return in a moment/second/minute, in a few moments/seconds/minutes.
This can be used in writing in present tense: He returns in a moment/minute. (I've never seen "second" used in this way, and I've never heard it spoken.)
None of my 3 search engines give any results for "He is returning in a second". I've never heard anyone use it (I learned British English as a child, and lived in various regions of the US and Canada). It'd be acceptable grammar; one can use the continuous form for something that will happen in the near future: I am meeting my friends in an hour. And there are a couple of people here who say they use it; I wonder whether they're both from other English-speaking regions. To me it sounds mildly wrong, probably because "return" in this sense doesn't feel particularly continuous, it sounds more like an instantaneous thing.
It would be impossible for Duo to allow all possible synonyms for all possible regions where English is spoken natively. I only ask them for corrections if they are clearly wrong.
In general I've found it is best to give literal answers here, even if the resultant English doesn't sound good to me. I do think Duo should also strive to be colloquial since non-English speakers take these courses, and it's not a good thing to teach them crappy English while teaching them good French. But that's difficult to do.
The system accepts all these: comes back/is coming back/returns/is returning/gets back/is getting back/will return/will come back/will get back/will be back/is back.
Reminder: what's best for Duolingo, also, is to write in full letters (dealing with contractions is very tricky).
I agree with jeanaux that we would say 'he is returning' and also that the second/minute thing doesn't matter, unless you where actually trying to be accurate. To be honest (in Australia anyway), we wouldn't really say 'He is returning' though. You will be far more likely to hear 'He's coming back' or 'He'll be back'.