What are the hardest grammar topics in Russian?

The usual tough spots: 1) The six cases—nouns, adjectives, and pronouns change endings. 2) Verb aspects (perfective vs imperfective)—which form to use when. 3) Three genders and agreement. They all interact, so it can feel like a lot. Many learners find it helps to build vocabulary and get the endings right in context before worrying about every rule. We teach these step by step in our courses.

For me aspects were the worst—when to say что делал vs что сделал. Still mess up sometimes but it gets better with practice.

Cases are learnable with practice and good materials. Don't try to memorise everything at once; use them in real sentences and your teacher can correct you.