[nfc]

It seems like an extremely coarse classification. Category 3 contains languages with very different degrees of difficulty, while Bulgarian and Russian are both Slavic they are nothing alike in terms of difficulty since Bulgarian is the most analytic of Slavic languages (has the less inflection). That makes it extremely easy to learn compared to Russian.

[vkazanov]

What is also interesting is how written Russian was heavily influenced by old Bulgarian. In fact, written russian includes a lot of older written bulgarian vocabulary.

This results in a weird paradox: for literate Russians it is easy enough to read written bulgarian but almost impossible to understand the spoken language.

[RugnirViking]

This happens in other languages too - danish and Norwegian are almost the same written, such that most products just combine the two on the packaging. But spoken it can be very difficult to comprehend

[vkazanov]

So... codified written languages are similar but real spoken ones have diverged? Is this only in the way things are pronouced or the differemce is deeper?

[optymizer]

I speak Russian and some Bulgarian as third/forth languages, and while I agree that Russian is more difficult, I wouldn't say Bulgarian is "extremely easy" in comparison. It's maybe ~20% easier at best.

[cpursley]

I think Bulgarian is considered the easiest Slavic language in terms of grammar because it has a simplified case system similar to how English dropped its cases over time.