"They are happy."
You are correct if you don't know the gender of the group or if it's mixture of males and females, but if you are talking about a group of girls that are happy then you should use 她們很高興。
Why does it need 很 when the translation is simply "happy" i was taught that 很 is an emphasis, as in "very happy" or 很好 ”very good".
This is true. I have flagged every time they use the character for"very" (hen) but don't state "very." In this question, I was correct to just use "gao xing" without"hen" because they didnt say "very" in the English sentence.
when speaking in chinese to a native speaker, saying "ta men gao xing" just sounds a bit plain, so "hen" is sort of used like an article of speech here such as "ma" at the end of a yes/no question would be. it's there to help the sentence flow better, to show more emotion, and to link the noun to the adjective.
"他们很高兴." means "They are very happy.", since "很" means "very". Very happy is not the same as happy.
This entry from the Tips and Notes should explain: https://www.duolingo.com/skill/zs/Greeting-2/tips-and-notes
他们高兴。also seems to be accepted here. Is it more colloquial, perfectly neutral, etc?
It is more neutral. 很 modifies adjectives to be more intense, in a way 'very.' People who know more, correct me!
Why don't we have verb (are) 是 here ? Why do we say 他们 很高兴 and not 他们是高兴 ？
他们 is a male only/mixed/unknown gender group and means "they" in English. 他 on its own is the male subject he.
你们 refers to a group that you are not included in and means "you guys" in English. 你 on its own just means you.
Like English sometimes the terms like "you", "you guys" and "they" are used depending on context and tense used.
How have you gotten this far and still haven't figured this out? I'm honestly curious.
Was it rejected for you? Apparently it was accepted a few months ago (cf. my earlier comment).
Noun + 很 + Adj.
The noun in this structure is the subject of the sentence. Sometimes the 很 (hěn) in this structure is translated as "very," but often it is just a way to link a noun to an adjective.
well, not necessarily. we use 很 instead of 是 for adjective. in this context, 很 means "is/are"