"Los árboles son verdes."
Translation:The trees are green.
My two cents: On one hand, the Spanish is "los arboles," and on another hand, all trees are not green. ;) A Spanish subject, when an uncountable noun, like amistad = friendship, often has an untranslated article. Here, unless we consider "los arboles' to represent all the trees in the world, I think we go with 'los arboles' = 'the trees,' since we seem to be talking about a certain group of trees.
So how would you say "Trees are green and roses are red" if "los árboles" cannot be translated as just "trees"?
Los arboles son verdes y las rosas son rojas. I did not say that "los arboles" cannot be translated as just "trees," I suggested that context matters. La vida es dura = Life is hard.
Thanks for the insight. So it seems to me that, lacking any context in the DuoLingo example, the listener has to make an assumption about whether the speaker is talking about trees in general or some specific trees.