Russian

Easier than it looks. The Russian Cyrillic alphabet has 33 letters, many of which look familiar (А, Е, К, М, О, Т) and behave roughly as you'd expect. The trickier ones are the lookalikes that mean something different (Р is 'r', Н is 'n', В is 'v') and a handful of new shapes (Ж, Ъ, Ь). Most learners read fluently within a couple of weeks.

Nominative (subject), genitive (possession, negation), dative (indirect object), accusative (direct object), instrumental (means, accompaniment), and prepositional (after certain prepositions, locations). Every noun, adjective, and pronoun changes ending depending on its role. The case system is what lets Russian shuffle word order for emphasis without losing meaning.

Every Russian verb comes in pairs: an imperfective (ongoing, repeated, in-progress) and a perfective (completed, single instance). Read 'I was reading a book' uses the imperfective; 'I read the book' (and finished it) uses the perfective. There's no progressive tense — aspect carries that work — and learners spend the longest part of the curve on getting the right one for the situation.

No. There is no Russian equivalent of 'a' or 'the'. Definiteness is conveyed through context, word order, and sometimes through the case used. Russian speakers learning English typically struggle with articles for the same reason in reverse — there's nothing equivalent in their first language.

It's mutually intelligible with Belarusian and Ukrainian to a fair degree, though the three are distinct languages. Other Slavic languages (Polish, Czech, Bulgarian, Serbo-Croatian) share grammatical structure and many cognates, so a Russian speaker has a head start, but they're not mutually intelligible without study.