"In winter you should wear a cap."
Translation:Zimą powinieneś nosić czapkę.
This verb is a so-called defective verb, it doesn't even really have an infinitive form. It's also the only, I think, verb that shows the subject's gender in the Present Tense.
Both versions are correct. "powinieneś" is used when you're talking to a man, and "powinnaś" when you're talking to a woman. Perhaps you had a typo somewhere and got corrected to another answer.
So here I have another dark area: When to use w and when not to.
(according to google)
In the morning: rano (nominative case?) in the afternoon: po południu In the evening: wieczorem At night: w nocy in January: w styczniu in Spring: na Wiosnę in Summer : Latem in Autumn: Jesienią In winter: Zimą
I had written "W Zimą" and was marked wrong, my question is, is it just a matter of memorizing and practicing or is there a rule which Duolingo has not made clear? As always, thank you for the time spent helping us.
The four seasons work in mysterious ways ;) Although I'm afraid that "w zimą" is definitely wrong, "w" usually takes Locative, sometimes Accusative. You used Instrumental.
To say "in spring/summer/autumn/winter" there are two correct variants per season: First is using the Instrumental form of a noun and no preposition at all - we can treat it as an adverb meaning 'in '. So we have: wiosną/latem/jesienią/zimą.
The second form uses a preposition and I guess it's better to just memorize them: "na wiosnę" (Accusative), "w lecie" (Locative), "na jesieni" (Locative), "w zimie" (Locative).
Months, like January, just take "w" + Locative.
"rano" (in the morning) is technically an adverb. You can also say "rankiem", which is the Instrumental form of the noun "ranek", but here it would also be an adverb.
"po południu" literally means "after the noon". The two words can be glued together to make the noun "afternoon": "popołudnie".
"wieczorem" is again the Instrumental form of the noun "wieczór" used as an adverb. And I don't think that's something that happens often apart from the words we discussed here.
"w nocy" is simply "in the night", Locative.