Fressen is the common verb for all 30+ million non human animate species eating so it spans the dining etiquette of pirañas, crocodiles, coral polyps, jellyfish, mosquitos, goats, phages, scorpions, vultures, whales, tube worms, bovines, felines, canines, reptiles, arthropods, & etc.
Both are correct.
Germans have two past tenses: Perfektum and Präteritum and there is NO difference like between English "present perfect" and "past simple". They mean completely the same, the only difference is that Perfektum ("hat gefressen") is used in spoken languague and Präterit(um) ("fraß") is used in written text.
You will encounter both later in the tree.
Frisst and fressen are for animals, and to use it for a human is an insult. So Duolingo will count it wrong if you use frisst and/or fressen with a human. For example, Er frisst Brot, would be wrong. Always use frisst and fressen for animals, NOT for people. For people, use isst and issen. You can use them for animals, but try to use frisst for animals ONLY, and isst for people.
For example, Ich issen Pizza (I eat pizza) is correct. Die Spinne isst Brot (The spider eats bread) is still grammatically correct (I don't think spiders eat bread), but it is best to get used to using frisst for animals and isst for people. DON'T MIX THEM UP! =)
There is more information in the Acc. Case skill on Duolingo. But "den" is used for masculine nouns in the accusative case, when it is directly receiving the verb from the subject. If a feminine or neuter noun is in the accusative position, then you would still use "die" or "das" respectively. The noun in the subject position would be in the nominative case, which is when you use der/die/das depending on the gender of the noun.
Yes, Käfer is both singular and plural. Den in this case is the masculine accusative definite article. Normally you use the nominative case articles when you speak about determining whether a noun is singular or plural, so it would be Der Käfer singular and Die Käfer plural
I answered 'the spider eats the beetle', which duolingo marked wrong. The correct answer was listed as 'the spider is eating the beetle'. Then coming to this comments page, it lists the correct answer to be 'the side eats the beetle'. I've run into this inconsistency before. Is it better in Duolingo to just always type 'is eating' instead of 'eats'?
Some sentences seem to lend themselves more to the progressive tenses, but I don't think this is one. The bottom line is that if your answer was on this page, it was their intention to accept it. Occasionally Duo justs rejects a correct answer, which should be this case. From time to time they make an error when they add a new accepted answer and somehow break an older one. I tend to always translate the present tense with our present tense unless it sounds really strange. Some of the languages I speak like Spanish have their own progressive tenses, although they are used much less often. But Duo and other programs try (with limited success) to reserve the progressive tenses in English for their progressive tense. So I am aware that they do occasionally only use the progressive, but not in sentences that sound equally good in both forms. The present tense is often used for narration, so sentences like this go with every nature movie you have seen.