"Winters here are very dark."
Translation:Los inviernos aquí son muy oscuros.
I had that as well but it was corrected to make oscuro plural. Oscuros
I'll go out on a limb and guess that this is a sweeping generalization about all winters in that area over all time and not about a particular day of a particular winter that might be different from the next day.
You are probably correct, Matthew, but I used "están" and DL accepted it. Oh, wellll . . . .
In Spanish, nouns cannot stand on their own as they do in English. You need an article (a, an, some, the).
The subject of the sentence always needs a determiner, and the verb of this sentence is "son," the third person plural of "ser."
Because 'oscuro' is an adjective, and needs the adverb 'muy' (very) to describe it.
Had you wanted to point out that there's a lot of darkness during wintertime, you could say, "En el invierno hay mucha oscuridad aquí." Then you'd be using an adjective (mucha, a lot) to describe a noun (la oscuridad, darkness).