Translation:Read both books before the next class.
"Read both books before next class" should be accepted. Duo thinks "the" is required, which is yet another shortcoming of duo English. When will you fix all these bugs, so we can concentrate on learning Russian instead of learning "Duo English"???
Where have you seen "the" next class? If I am not mistaken we say " next year next week next time" and there are no definite pronouns in Russian.
It's partly a question of idiom and partly one of specific vs. general. "The next class" is very specific - it's not stated, but classes are schedule, so one would know exactly when the next class is to start and how long it will last. "Next week/month/year/century/etc." are vague and general - we may know when it will start and how long it will last, but we don't know whether something would be due precisely by the time the week/month/etc. starts, or some time during that week/month/etc. For example, in a business setting, getting something done before next week may mean before Monday (the 2nd day of the week) or by the next time for a progress report, which could be Friday; "next week" is not as definite as "the next class".
Accusative feminine (it could also be nominative, but not in this sentence). Masculine form is «о́ба». They look strange because they were dual in the past; now, we don't have dual number, but this word retained its old ending so it looks different from other pronouns.
Thanks. I forgot about the dual. So it is like два then? And is there also a neuter form?
Actually, it's declined like «двое» and not like «два» (althoug they're definitely related). You can see the declension table in the Wiktionary.
"Read both books before the next lesson"
This is counted as incorrect. Why isn't Russian as intuitive as the other languages?
This was one heck of a sentence to try and decipher from the audio. Not because the audio is bad, just cuz my Russian stinks.