"The autumn was cold."
Translation:Осень была холодная.
Hmm, that's what I've written as a translation for the autumn was cold. I think it's technically correct, so I'll report it - but it's probably not the answer they're looking for so I'll it to the mental list of questions that I just get wrong every time lol.
Probably more like "it was cold in autumn" than "the autumn was cold". It seems to me that Duolingo is rather consistent with this type of sentences.
What case is холодно in here? And do you always use a different case for adjectives in the past?
Cases don't depend on a tense. Осень была холодная, осень - [есть] холодная, осень будет холодная, all adjectives (холодная) are in nominative case as the defined subject (осень).
You mentioned "холодно" which is not in the answer here, so I suppose your answer is different. Холодно is an adverb derived from холодный, they are not declined by cases. Not sure what answer you had.
Weird I'm very confused - that was what I expected (холодная) but when I wrote this, the correct answer was showing up as осень была холодной
Осень была холодна, осень была холодная (now displayed as the main translation), осень была холодной, all these are the same. Была/будет can take an adjective in instrumental or nominative case, or a short adjective (first). Instrumental case is much more common here though.