Winter mountain is correct in the literal sense, but I wouldn't say winter mountain is a normal expression in English. 冬の山 is a common equivalent for the mountain in winter. Things in Japanese can't always be translated so literally.
Fuyu no yama is understandable in Japanese and if translated to the best possible English means the mountain in winter. However, this whole program has been based on literal translation so it might be wise to reconsider this sentence combination.
So can I phrase it like: 冬には山が危なかったです。? Because to me the thing that’s dangerous is the fact that it’s in winter. I feel like that should be the subject. I mean obviously duo won’t accept it...but any other officianados out there who can verify my sentence?
That would be "Winter (on/of/at) the mountain was dangerous" but the dangerous thing, in the English sentence, was the mountain (in winter), and the primary noun should probably be preserved in translation.
The English sentence is written in the past tense, but so far as I understand it, the Japanese sentence is in the present tense. Or have I just completely misunderstood how past and present tense is expressed in Japanese?
I believe with some adjectives the way they end makes it past tense. I think for certain verbs if it ends in -katta desu, then it's still past tense, but for others, deshita is the correct form? I could be wrong though