I also have a hard time understand this: why?! It is if the German course would teach dated the dative -e ending for masculine and neuter nouns (zum Tage, auf dem Tische) or if the Swedish course would teaching the dated way of conjugating verbs in plural (vi äro, vi gingo till skogs etc.).
According to Google Ngram Viewer, the usage of со мною has dropped ever since the beginning of the 20th century and is still dropping in favour of со мной.
With all this being said, I actually like being ‘old-school’, using slightly dated terms and expressions, just as ‘homewards’ in English or ‘den gamle mannen’ in Swedish.
The audio contains a grammatical error "со мною" -is incorrect правильная форма " со мной" as in Instrumental case: ( Nominative Case Я, Accusative Case Меня, Genitive Case Меня, Dative Case Мне, Instrumental Case Мной, Prepositional Case Мне)
This is not technically an error (notice that it's represented in the text as well). "Мною" is an older form of "мной" that is used less frequently now, but still valid.
В современном языке это архаичная,или просто неграмотная или диалектная форма. Вы найдете ее в литературе 17 го века или услышать сегодня в сибирских деревнях от малообразованных людей.
The real question is why would you teach an old form, the equivalent of Shakespearian English, that sounds pretty funny (режет ухо) to the people who are just starting to learn the language?
Listen, it has never been in acceptable form in my lifetime, which is pretty long.
Thank you so much for your explanation. It's difficult to understand why Duolingo would use an older form of "со мной" but they love to irritate people taking this course.
Correct or not is an open question but I'd also say that this shouldn't be taught to foreigners. You'll never hear an educated person say that unless it's meant to be poetical or smth.
I skipped the bulk of the beginning of this course, so it's possible I missed something, but unless there's a lesson on slang or verse, I would remove this from the course. Pretty much the only time you will hear this in my opinion is in songs, when they need to make something rhyme or meet a particular meter.