By the way, this word, b1tch, is not a rude word in English - at least not in modern times.
Oh yes it is.
The three most common mistakes mad by Poles learning English are articles, verb tenses, and vocabulary. I won't give you any examples because I could write all night, but I think you-especially teachers-know what I mean.
I start emphasizing articles at the pre-intermediate level. With a lot of use, most students get pretty good at using them, but they're difficult because they don't exist in Polish. I had a post FCE student tell me he didn't use articles because they weren't important. He changed his mind when I asked him if these two sentences meant the same without an article: I speak little Polish. I speak a little Polish. His response? Hmmmm. I see what you mean.
OK, I understand that people are going to make mistakes with verbs, but the two that make me crazy are: I going (and any other continuous form), and he speak. The latter is so easy to learn that I just don't understand why they make that mistake. When I hear these, I insist that they say I'mmmmmmmm going and he speakssssssssss until they use them properly. I seldom have to resort to violence to get the point across. :)
When I say vocabulary I mean either not knowing the word or using the wrong one. False friends are classes. My friend is very sympathetic (nice), etc ad nauseum.
But it's all good. The point of learning another language is to communicate, and we're all going to make mistakes in a language that's not our mother tongue. If people can understand what you want or mean, even if you say it imperfectly, you're communicating, and it only gets better with time.