What, exactly, did you type?
"The bear eats the bananas" and "The bear is eating the bananas" are both accepted in German-to-English translation exercises.
"The bear devours the bananas" and "The bear is devouring the bananas" are accepted alternatives but are not required.
Perhaps you made some other mistake? Could you post a screenshot of the error page, please?
For example, "The bear eat the bananas" and "The bear is eats the bananas" are both wrong because the verb isn't right in English.
Because there's no reason to use the dative case here.
The bananas are directly affected by the bear's eating (they are disappearing into the bear's stomach) and are the direct object of the verb fressen.
Direct objects almost always take the accusative case, and that is the case they are in here.
"Whom or what?" is the question for accusative, so it's *Wen oder was frisst der Bär? - Die Bananen."
The question for dative is wem? (to whom?).
English merged the dative and accusative tenses into a single objective case, so the fact that some of the pronouns look similar to German dative pronouns (e.g. "whom, him, her" look more like wem, ihm, ihr than to wen, ihn, ihr) doesn't necessarily mean that the German equivalent will be in the dative as well -- it could be in the accusative.
Frisst is eating like an animal, and is used for when animals eat.
Frisst is the equivalent for isst, and the rest of the verbs can be converted from eating like a human to eating like an animal by adding "fr" to the beginning of the verb, like isst to frisst
This is also explained (at least on the desktop version) underneath the practices in Animals 1.
Hope this helped :)
Those notes just give general rules, which don't cover every single noun.
The plural of der Bär is die Bären.
(Also, it's a masculine weak noun, so it takes -en in all cases except nominative singular -- it's Bären not only in all cases of the plural, but also all other cases of the singular: der Bär, des Bären, dem Bären, den Bären.)
Fressen and essen can be tricky for English speakers. People generally "essen" and animals usually "fressen". In that context, each word is neutral. People sometimes use "essen" to describe a domestic pet eating, but that is rather anthropomorphising. People sometimes describe the eating habits of another person as "fressen", and in those circumstances that person is imagined as bolting or gobbling his food in a messy or greedy manner. If you really want to get the meaning "devouring" (eating massively, greedily, and quickly; shovelling it down) then perhaps the verb verschlingen would hit the mark. I'm not sure. Would a native German speaker care to comment and shed some light?