When can we learn new words? I keep practicing the same words over and over again and sometimes it makes me "Crazy!" If we go to the next level, can we then learn new words?
No, you cannot learn any new words on higher levels. You might however, learn new words by either trying the Labs or the Reverse Tree.
I have the same problem. I cannot understand why I have done literally 20x practice sessions on the same like 8 words. I am about to leave Duolingo because I cannot find a way to get it to teach me new words.