The sentence "they find" sounds very unnatural in English. The sentence "they (verb)" implies that people do that verb all the time, as a habit or quest, as part of their identity. They work. They play. They search. They love. Those are fine, and sound very powerful. "Find" sounds strange with this usage. They find ....what? Are they very lucky people who just happen to be able to find things all the time that the rest of us cannot find?
They were just sitting there by the tree in the beautiful garden, goofing around like loved ones do. But suddenly: They find ! An apple fell from the tree. Adam noticed his stomach was growing, he had been preoccupied with godly matters all afternoon and had thus forgotten to eat. So he firmly said to Eve: Let us mangeons une pomme rouge.
you cannot tell the difference in sound between il and ils, or elle and elles just by listening; you cannot tell whether the verb is singular (trouve) or plural (trouvent) by listening either, as all those plural endings should not be sounded. The only way you can tell what it should be is by seeing the written words or knowing the context. As you cannot here, Duolingo should accept bothl answers : il trouve and ils trouvent are both correct.
I don't agree: you should hear the "ve" both in "trouve" and "trouvent;" it is only the last 2 letters in "trouvent" (n and t) which should not be heard; In fact both words should sound the same. Duolingo often sounds endings or makes liaisons when it should not, specially when it repeats the sentences slowly.