you’ve been corrected on your spoken russian. i don’t think it’s nessesary to repeat what’s been corrected.
i guess you did not read some endings correctly or have no idea how to say them correctly. but that’s alright, you need someone to correct you on speech.
it’s not so much grammar since you said you read it, i guess it’s spoken, and comes off like grammar.
many people can’t speak endings well. since english does not use many endings on words, may be like s, ing, ed, that’s about it. while russian language has a lot more engings and that’s because russian language does not use a lot of words like of, the, a, etc may be i am not exactly right but something like that.
i did try to teach some russian on language exchange, the most fun part was trying to make them say russian words correctly.