The parts of English that trip up Polish speakers
Articles come first, because Polish simply does not have them. There is no equivalent of a and the to transfer, so they have to be installed from scratch rather than corrected, and they stay fragile long after the rest of your grammar is solid. A teacher who has taught Polish learners knows this is a systematic gap rather than carelessness and drills it deliberately, instead of quietly adding a missing the to every third sentence and moving on.
Tense is the second one, and it is mostly the perfect. Polish maps time differently, so "I work here since 2019" and "I am working here for three years" feel entirely logical, and both come out wrong. The same goes for the past simple against the present perfect: whether something is finished, or finished but still relevant, is a distinction English marks with grammar and Polish handles with context and aspect. It is learnable in an afternoon and takes months to become automatic, which is exactly the kind of thing regular 1-on-1 practice fixes and a self-study app does not.
Then the false friends, and Polish has some expensive ones. Eventually is not ewentualnie, it means in the end, so "eventually we can meet on Tuesday" says something quite different from what you meant. Actually is not aktualnie but in fact; currently is the word you wanted. Sympathetic is not sympatyczny, a dress is not a dres, a fabric is not a fabryka, and a chef is not your szef. Nobody corrects these for you in an email thread, which is why they survive for years.
Pronunciation has its own short list. English th has no Polish equivalent and usually surfaces as t, d, f, v, s or z. English w is not Polish w, which is why wine and vine can merge. Polish devoices final consonants, so bed comes out closer to bet and dog closer to dock. Polish stress is almost always on the second-to-last syllable while English stress moves around unpredictably, so words like develop, comfortable and photographer need to be learned one at a time. And English contrasts long and short vowels in a way Polish does not, which is the whole ship-versus-sheep problem.
Finally register, which is where fluent speakers still get caught. Polish signals respect with pan and pani; English signals it through phrasing, hedging and indirectness instead. Translate a perfectly polite Polish request word for word and "Send me the report today" can land as an order, while the safe-looking "Dear Sir or Madam" reads as oddly stiff to a colleague who signs off with his first name. Knowing how blunt you sound in English is a skill of its own, and it is best learned from somebody who will tell you.