After a few years of learning, it's too late - most learners have fossilized bad usage so they have to unlearn a lot of what they think they know (and unlearning is harder than learning).
for 99% of all people who claim to speak some polish, this is exactly the case. they learn polish and disregard all declensions, proper usage of the polish numerical sytem, gender, miejscownik, which verbs require which case, etc. etc. and in the end, their Polish is crap. my first year in Poland i did nothing other than study basic words and grammar grammar grammar till I puked from grammar. in the end though, what it gave me was a grammatical basis for all new words I learned thereafter. After 3+ years in Poland, my vocab is still small but my grammar allows me to adopt new words and use them rather quickly. The hard part is mostly over and now I mainly study vocabulary for the grammar comes quite naturally now, aside from maybe counters of words I haven't heard yet.
Poland generally has the ów or, to a lesser extent, the ek ending for 5 upwards to 20 I think. Jedna butelka, dwie butelki and sześć butelek. Jeden widelec, dwa widelce i sześć widelców etc etc. You don't need to think about the endings all the time like in Japanese.
saying "you don't need to think about the endings all the time" and "Polish" in the same sentence is an outrage.
I don't know a lick of Japanese, but if I had to guess, I doubt that when you say "with nihon" or "on nihon", or "i don't have "nihon", it's still "nihon", every time. not the case with good ole' Polish.
2 bottles = dwie butelki
with 2 bottles = z dwoma butelkami
on 2 bottles = na dwóch butelkach
I don't have 2 bottles = Nie mam dwóch butelek
You can never take a number for granted because at any moment while in context, it can change which not only makes speaking properly difficult, but comprehension tough because you need to get used to hearing someone say the exact same damn word but in several different forms.
I'm sure you've seen the web page discussing how Polish is the hardest language in the world where the author displays the "17 ways of saying the number two" in Polish. The kicker is there are 17, not including case changes, meaning he doesn't list drugiego, drugiej, drugim, drugich, drugą, etc. etc. putting you well over twenty forms.