Why is there both "to" and "jest" in the sentence? What does the "to" signify?
Well, although „to” can be translated as "is" in certain sentences, it isn't actually a verb. „To” means simply "it". So a sentence like „ryba to zwierzę” literally translates as "fish – it (is) animal”. It might sound primitive that way, but it's perfectly valid Polish sentence.
So when you see such sentences with „to” but without „jest”, the latter word is still assumed to be there. You can also include „jest” and that's too perfectly fine.
This also explains why you would negate sentences with „to” as „to nie” rather than „nie to”.
Ohhh, wow. So the lack of jest in other sentences is just some kind of idiomatic shortcut and they're in fact the exceptions to the rule? Why then do you not need to use the accusative for sentences of form "noun1 to noun2" but it is being used here? (Sorry, I'm not trying to argue that I'm right or anything - clearly I'm not. I'm just still a bit confused as to how this thing works.)
Both sentences with „to” and with „to jest” use nominative for both nouns. Indeed, this sentence uses nominative for both nouns. Only when there isn't „to”, but there is „jest”, would you use instrumental for noun 2. Then the sentence would be „Jesień nie jest dobrym okresem”.
Thank you so much! Last question, I swear: does "nie" change the case of the second noun in "to" sentences?
As Duolingo is an American company, the courses here have to choose the American versions as the default ones. British ones should always be accepted anyway.
Seems like a more common sentence, but it's not what 'okres' means. 'okres' is a very vague word, 'season' is too specific.