"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.
The subject of the sentence always needs a determiner, and the verb of this sentence is "son," the third person plural of "ser."