I disagree. I'm a native English speaker. In the sentence "it is boring in the evening," the "it", unless otherwise specified, can only be assumed as "the evening". Not only is that a very clunky sentence in English, but it is also exactly the same as saying "the evening is boring," as the "correct" English version is semantially "the evening is boring in the evening". I'd recommend this question be removed, I'm afraid.
You do, but that does not make it valid. There is too much translator-think here, so learning to use the language often gets pushed into second place. I am not a translator, I am trying to be a language user, so I am not interested in any kind of purity of grammatical comparison between two very different languages.
You could easily say "Wieczorami" to say that generally evenings are boring (let's say 'here at this camp the evenings are boring'). But "wieczorem" could just refer to this time of the day generally and it could work for this context. It would still be better if it wasn't suggested to you...
I am still unconvinced as 'the evening is boring' or 'evening is boring' cannot be judged as too far away grammatically, because EN grammar is not a subset of PL. EN has to be as far away as it needs to be. My job is to edit transkations, and many of the persistent are those where the translator never lets their native grammar go.
OK, good point. Speaking for myself, I am worried at times that I am trying too hard to "speak English using Polish words," or trying to fit Polish into an English paradigm. I guess the thing to do is to refrain from trying to force our customary English patterns of saying things onto Polish, and instead learn to use the Polish ways of expressing things when we are speaking in Polish.
So, your larger point is taken.
I read the entire discussion and see no explanation that makes sense to me for why "the evening is boring" is wrong. I wrote "Evening is boring" and that was also marked wrong. What part of speech are wieczór and wieczorem? To me, "(the) evening is boring" and "evenings are boring" are equivalent phrases in English. Perhaps using the singular evening is an example of synecdoche, using a part to represent the whole. https://examples.yourdictionary.com/examples-of-synecdoche.html
It's difficult to force one to understand this Polish sentence by using English grammar. Wieczorem is an adverb. "In the evening" is an adverbial phrase, a phrase used as an adverb. Adverbs modify other adverbs. Boring/nudno is an adverb. So "in the evening"/wieczorem modifies "boring"/nudno. Your sentence uses the noun "evening" as the subject of the sentence. In Polish, that would be Wieczór jest nudny. Notice that nudny is an adjective not an adverb, modifying wieczór. The meanings are so similar even though the grammar structure is different.