Another incredibly tricky one for Slavic languages is imperfective vs perfective aspects. In most other languages, perfect vs imperfect is just a standard construction. In Polish (and Russian, though my examples here are Polish), you change the verb itself. How? Well, sometimes you put "po" in front of it, like rozmawiać / porozmawiać. Sometimes it's "z" (jeść / zjeść), sometimes "u", "na" or "wy". And sometimes you just give up, like oglądać (but obejrzeć in the perfective), widzieć (zobaczyć), mowić (powiedzieć).
(For this reason I have found it very hard to progress in Polish without conversation with native speakers who aren't too polite to correct me.)
36 isn't that bad but Russian also has 253 irregular verbs [0], each of which adds another set of combinations. A lot of them are just small tweaks and there are some that follow a set of patterns (e.g. идти with its various prefixes) but it still adds a lot of overhead.
[0]: https://en.wiktionary.org/wiki/Category:Russian_irregular_ve...
Modern Russian has 6 cases (with a couple more that pop up very rarely) with 3 genders and singular/plural endings. It's a total of 36 possible combinations, which is not that bad. Masculine and neuter also share most endings so there aren't actually 36 unique ones
Probably the most difficult thing about cases when I was learning Russian was remembering which verbs took a different case than I would expect, e.g. dative instead of accusative for what seems like a direct object.