"Spring is not a dry season."
Translation:Wiosna nie jest suchą porą roku.
We don't really use "sezon" for "season of the year".
"Sezon" is Season 4 of "House of Cards", or "2016/2017 Premiership season", such things. Or "sezon wyprzedaży" (sales season?) in shopping malls...
In the positive form of the sentence, instrumental would be used. The negative form doesn't use genitive because the positive form doesn't use accusative.
I submitted "Wiosna to nie suchy pora roku" - an incorrect answer which was marked as correct with typos. However, the answer suggested to me was "Wiosna to nie sucha pora roku", which is in the nominative case. I don't understand why the nominative would be used here.
It's because the verb "to be" is, in your example, and in the suggested correction, represented by the word "to", which, unlike "jest", doesn't take Instrumental.
I think my sticking point is why isn't it in the accusative? It's the subject of the verb, isn't it?
Well, actually, it's the object of the verb, and would be Accusative in some other languages, although "to be", specifically, is often a special case anyway. However, "to" uses Nominative for both the subject and the object. See more, by Jellei, one of our esteemed Mods, at https://forum.duolingo.com/comment/16373167/A-guide-on-X-is-Y-and-This-is-Y-constructions
The translation should not include 'roku' as it doesn't include it in the English.
The translation has to include "roku" because "pora roku" is how Polish refers to "a season" (spring/summer/autumn/winter).
Generally "yes", although it's some specific time, like in "it's time to go to sleep" or "this is not a good time". After all, the main translation of "time" is "czas".