In English you don't say "to eat fruits", although i don't think it's incorrect to say it like that, just unusual. When you have an uncounted amount of fruit it's "fruit" in the plural, and "piece of fruit" in the singular.
"I am eating a bowl of fruit."
"I am eating a piece of fruit." (Can be a divided piece of one fruit, or one whole fruit.)
"They are having a sale on fruit at the market."
"I got this piece of fruit on sale at the market."
However "There are four fruits in my bag" is correct, and "There are four fruit in my bag" is not.
Compare these two sentences:
- We are eating (some) fruit = some non-specific fruit is being eaten, a generic statement on the type of food being eaten
- We are eating the fruit = refers to a specific piece of fruit or a specific bowl of fruit
Note, "fruit" is usually a collective noun that takes a singular verb:
- Fruit is tasty. = a general statement about fruit
- The fruit in that store is expensive. = specific fruit is expensive
The plural "fruits" exists. It is not used much except when referring to species/varieties.
- The citrus family (Rutaceae) is the source of many edible fruits.
- Botanists collect flowers and fruits in order to study them.
- slices of various citrus fruits
You can say:
- I am eating a piece of fruit, NOT I am eating a fruit.
- I am eating a vegetable.
- I am eating (some) vegetables.
It's used as a mass noun here, which is uncountable and so does not need an article.
It is like other mass nouns such as "rice" or "water" or "sand" where we do not say "We are eating a rice and drinking a water". We simply "eat rice" and "drink water" and can also "eat fruit".
The German word Obst is similarly a mass noun and so does not take an indefinite article.
Why is it not we are eating fruits?
Because Obst is "fruit" in general -- the "substance". Not necessarily individual, complete fruits (= Früchte).
For example, if you have eaten half an apple and a quarter of a pear, you have "eaten fruit" but you have not "eaten fruits" (because you did not consume a whole apple nor a whole pear).
How do you know the difference between I am eating and I eat.
Without any time expression such as "every day" or "right now", you can't tell the difference -- German doesn't make this distinction grammatically.
Both translations are thus generally accepted.
why is it we are eating fruit wrong
It isn't wrong.
Do you have any evidence that that translation is rejected by Duolingo? A screenshot, for example?
I didn't take a screenshot
Then we will never know exactly what happened.
My guess, though, was that you had a listening exercise ("Type what you hear", i.e. in German) and that you translated into English instead.
And that Duo did not say that
the correct answer was "we eat fruit"
but instead that the correct answer was Wir essen Obst., which can be translated as "we eat fruit".
In German you would say: "Wir essen Obst." or "Wir essen Früchte."
Both sentences are possible, but they mean different things.
- Wir essen Obst. = We are eating fruit. You are using "fruit" as an uncountable noun, as a kind of "material". It could be chopped-up fruits or whole fruits or anything.
- Wir essen Früchte. = We are eating fruits. You are using "fruits" as a countable noun: individual entire fruits. For example, one whole apple and two whole pears.
When reporting an error, it's best if you
- quote the entire sentence you are referring to (nearly all sentences have multiple accepted translations, so talking about "the translation" is not specific enough)
- say which part of it is wrong
- say why that is wrong
- say what it should be instead
why "we are eating fruits" is not right?
Because the German sentence isn't talking about individual, entire fruits (= Früchte) but about "fruit" as a substance (= Obst).
For example, if you eat three apple slices and half a banana, you have eaten fruit (Obst) but you haven't eat fruits (Früchte).