You have translated into the past tense, but 'Zij komt' is in the present tense, and you have left out this verb. - As for the weirdness, imagine that the girl has had an argument and didn't want to swim with the group. Now she has decided to come, but just one time. (Maybe more will follow.)
"Een keer" is a typical Dutch way to express a fairly vague, open-ended time frame, either in the future or in the past.
We use the verb in the present tense, followed by "een keer" to indicate a general intention, not a specific occurrence or set date in the future. In this particular case, it means something like: "She will drop by for a swim some time soon-ish". Another example: "Ga een keer fietsen!" is a general admonishment to take the bike (at some unspecified time in the near/intermediate future - possibly as a healthy alternative to the car).
I agree with you: normally once (when used as a time adverb, not as a conjunction) refers to a past time. More info: https://dictionary.cambridge.org/grammar/british-grammar/adverbs-of-time-and-frequency/once
Every time I come across this exercise, my heart sinks. So many people are confused about what it actually means, but there is no comment from the Duo team to help people interpret what to many people is a translation that doesn't make sense. Is it the case that this is just a Dutch phrase that doesn't translate well to English?
the difficulty is of course the part : ...een keer... . If you translate that literally to english it means: ....one time..... But in normal day dutch we understand it as: ....sometime or maybe never.... It is not a contract, it is a vague promise, it is an intention. maybe like the spanish .. tomorrow. Someday, when we are in a good mood, and the weather is nice, she will come over to swim. And for that promise, or intention there is no good english translation, although: "some time" is already a good fit, but i don't know if Duo accepts that. Duo's translation is : "once" and not "twice", but that misses the clue completely.
It's interesting to note that in Belgian French we have the exact same thing ("une fois"), which French people don't really understand (and they make fun of us for saying it). It's the same meaning as what you've described, but it's a belgicism. I imagine we got that from Dutch, since we're so close and both languages in Belgium are influenced by each other.
As a native speaker, "een keer" is used to indicate an unspecified moment in the future. "Een" is pronounced then as "un" whereas if you nean only one time, "een" should be pronounced as één (as in "ain't") we use this first version to indicate that someday in the future we will do the action mentioned, without committing to it. Like: "jullie moeten echt een keertje langskomen" (you really ought to come visit sometime) in this case, i think the correct english translation should be: "she is coming for a swim sometime"
To my American ears, "she comes to swim" or just as in Australia, "she comes for a swim" indicates purpose. She is coming over to a pool for the purpose of swimming. "She comes swimming" would not only sound strange to me, but would indicate manner, i.e. she came to me by swimming, instead of by boat or hovercraft or something else. Now, "she is going swimming" would probably indicate purpose to me, but then, we use "to go" as a sort of modal in a way, whereas we do not use "to come" in the same way.
Because it means a instead of one.
Note that the pronunciations of een and één differ -- the former sounding something like un or 'n and the second like ayn.
I have a little disagreement with the English translation - "één keer" literally means "once" or "one time". She is planning a single visit and no more. But to my ear, "ze komt een keer" means she is going to drop in sometime, without any thinking about how often it might happen. It is a common expression in Dutch. "Kom eens een keer langs!" = "Come and drop by sometime!" It certainly does not mean just once and that's it.
yes, that is okay. Just to be sure i checked google translate. There it is a little bit different: https://translate.google.com/#view=home&op=translate&sl=nl&tl=en&text=komt%20te%20zwemmen Google says: Komt te zwemmen = "comes to swim". But that is basically the same as "will swim".
Your translation is better than Duolingo's in my opinion. I think it is OK... with the comment that the Dutch means sometime, or maybe not even at all. I guess your version could mean that too.
And re the accents, those indicate different sounds in Dutch. The è is a short e, whereas é is a long Dutch e (sounding like our long A) - similar to their usage in French.