Translation:I explained it to him but he didn't understand.
There isn't one. They probably include it in the English translation because at least for many English speakers and dialects when the verb explained is used in a context such as this one (a transitive verb within an independent/main clause) it generally occurs with an explicitly encoded object (it in this case). It's a conventional, entreched/fixed expression, which might sound a bit odd or unusual without an object (in this particular case). That said, the usage of explained could vary somewhat depending on the context, speaker, and dialect. Anyway, I'd assume that's why the course contributors included 'it' in the translation. You can appears report it if they made an error it omitted something. Also keep in mind that Hebrew and English syntax and grammar won't always match up perfectly.
It showes incorrect answer and the same sentence it suggest as the correct one.
Wrong again. Please could you fix that? For suite a number of sentences the audio does not correspond to what is written.
As far as I know you explain something to somebody OR you explain somebody something
I see that others are having the same difficulty with some of the audio matching the sentences. The question is, do folks from Duolingo actually respond and correct the problems or are we singing in the wind? This problem keeps coming up in some of the sections and it become quite frustrating. I'd be interested in hearing from the company.