I'm not that far in my Esperanto course, but I did stumble upon an incorrect answer of mine when I tried to answer the question, They are reading at night. (or something like that). Anyway, I corrected it, or so I thought and I put down, Ili estas legas cxe nokto. Then it said I was wrong and I sort of got why because I put in the "is", but then I saw the translation and they said it should be Ili estas leganta cxe nokto. Can someone explain?
I sure hope it didn't really say Ili estas leganta because it should at the very least be ili estas legantaj.
The way the course works is the course administrators can enter in multiple "correct" answers. If you enter an answer that is not on the list, the system will present you with the answer that it thinks you're trying to say.
My guess is that one of the admins entered in the answer you were presented with and it's the only one with "cxe" in it. If that's the case, they should remove the answer since "cxe nokto" is wrong.
What should it be to say at night. (I meant to add the j, sorry) and what does leganta mean?
Leganta is the present participle form of legas; it's the difference between I am reading, and I read. However, while this form does exist, it's used much less in Esperanto than in English, and you'll use the simple present, legas form much more often, even when English uses the participle form.
What should it be to say "at night"?
You could say "nokte" (the adverb form of "night/nokto").
You might also be able to get away with "nokton" (the accusative form of "night/nokto"), but then it might confuse some who read it as "They read a night." However, most should understand what is meant.
Another way you could say it is "dum la nokto" ("during the night"). It's a little wordier, but you probably won't have to worry about it being misunderstood by beginners.