Interesting, translation "this is my foe" gets marked as incorrect here, while in other exercises 'foe' was a valid substitute for 'enemy' in translation of wróg. Why?
Enemy is just much more common and foe may be omitted as a possible translation in some sentences. You should report such cases.
I am not sure why there is such a focus on wróg. As is usual with polish courses there is too much emphasis on all the possible cases over structure. I live and work in Poland, but when am I going to need wróg or other words when there is nothing yet for shopping even. It is like learning English by learning all the tenses first. I even ask people at work where I could use words for enemy, friend, judge, lawyer, etc in the office, and they don't know. I cannot even ask for a cup of coffee.
I hear what you are saying too. I've been using Duo for a while now, and am married to a Polish woman. I have a fairly good understanding of grammar and cases but still can't hold a small conversation. It is frustrating. What I have started to do recently is use other resources such as, film, audio books, and other apps to supplement my learning further. Tiny cards is another great app that has been amazing for me so far, and has improved my vocabulary extensively.
Why is "it is my enemy not accepted" here, i remeber having read here that to more like "it is" than "this is". I guess that in normal life speaking this will not be a problem, but I really would like to understand the difference. thanks
From the point of view of Polish, "This is", "That is" and "It is" are absolutely the same. "It" is an accepted answer, it should have worked.