Translation:China's Yellow River is famous.
很: The most frquently used adverb of degree, indicating a rather high degree. Sometimes it is obligatory, is pronounced in a neutral tone, and nearly loses its effect of showing high degree. When 多 or 少 is used as an attributive and if there is no other adverb before it, it must take 很 as a modifier with 的 being optional. When an adjective, especially a monosyllabic one, serves as the predicate of a sentence or the compliment of a verb with no other verb before it, 很 is necessary. [汉英虚词词典 - A Chinese / English Dictionary of Function Words]
Great. And so, Duolingo should always accept English both with and without "very" when the Chinese has 很.
And if there are cases when both aren't acceptable then that should be a specific grammar point they focus on, outline to us, and give some detail on why, when marking us as wrong.
In the meantime we must all click to add missing translations when both are not already accepted.
It also depends on the situation, but for a lot of the situations in which Duolingo doesn't accept both, they really should.
As to that, it's difficult to explain since they are either context based, where "very" is optional, or specific-situation based, where there are commonly used, unwritten rules of translation, hence the best way is to read and hear more.
Finally, “very" can or should also be used in other places where ”很“ is not present, depending on the word, phrase or sentence.
My question exactly. And how is one to know when "very" is intended?
It looks like they changed the "Tips and Notes" since this course was first released. The former "Tips and Notes" can be found at https://duome.eu/tips/en/zs. Under "Greeting 2", it explains clearly why 很 means "to be" in sentences like the one here.
"When we want to describe something we don’t use the verb “to be”, we have to put something else before an adjective. The most common word used to do this is 很 hěn, which literally means very. It goes between the subject (who or what we are describing) and the adjective (what we are describing it as). Since 很 is used so often like this, it oftentimes doesn’t mean “very”, just a link between a noun and an adjective."
中国的黄河 is the subject (what is being described) and 有名 is the adjective (what the subject is being described as). Putting 很 between the two, we get 中国的黄河很有名 meaning "China's Yellow River is famous".
(I rather prefer the older "Tips and Notes"). It's too bad they changed them. The LingoDeer app also has very good grammar explanations.)