Video is incorrect. Russian is South Slavic language. It's much closer to language used in Montenegro (which is South Slavic) than to Ukrainian or Belorusian languages. Russians have troubles with understanding of something written or spoken in East or West Slavic languages, while they have much less trouble with South Slavic languages. It's because Russian language is based on Church Slavonic language[1][2], which was develop by Saints Cyril and Methodius[3] using South Slavonic dialect spoken in (now) North Greece.

Also, samples of history are incorrect, causing multiple LOL's.

[1]: https://en.wikipedia.org/wiki/Saints_Cyril_and_Methodius

[2]: https://en.wikipedia.org/wiki/Church_Slavonic_language

[3] https://en.wikipedia.org/wiki/Old_Church_Slavonic

You obviously know a lot more about this than I do. I don't think we are arguing about anything. The Langfocus video mentions the 2 saints and Church Slavonic.

There are many debates on youtube about how much of 1 slavic language a speaker of a different slavic language can understand.

I was recently listening to an episode of Sean's Russia blog that briefly discussed how much Russian is spoken in Ukraine. http://seansrussiablog.org/2015/04/09/post-maidan-ukraine-an...

I was listening to an episode of This American Life recently, the interview subject, who sounded Greek to me, was repeating the Old Slavonic liturgies he sang in his youth during Byzantine Orthodox services.

I'll have to read and find out if Old Slavonic and Church Slavonic are different or the same thing.

Please note, that video supports my point: between 500BC and 1600BC, Russian language was non-existing. It's why video jumps from 500BC (proto-Slavic language) to 1400BC straight, skipping 1K years, 2/3 of history of Slavs.