NOT A TEACHER
Hello,
1. "Really" is my favorite word in the English language.
2. Some years back, at another grammar helpline, I asked so many questions about that word that the website's owner very gently asked me to stop posting questions about what word was modified by that adverb in particular sentences (for there is often a difference of opinion).
3. Since this is a discussion forum, I would like to share my opinion, which could be completely wrong, of course.
4. "Mona is nice," "Mona is not nice," and "Mona is not really nice." From what I have read, the third sentence is midway between the first two sentences.
a. In other words, Mona poses as a nice person, but people who know her well realize that she is a phony.
5. "Mona is really not nice" means to me something like: "I am telling you the truth. Please believe me: Mona is not nice!" In other words, it might be restated as "Really, Mona is not nice" or even "Mona[,] really[,] is not nice."
*****
Raul knows a few dozen phrases in the Chinese language. Nothing more. One day he says something in Chinese to Mona (who speaks fluent Chinese).
Mona: Wow! I didn't know that you speak Chinese.
Raul (chuckling): I do not really [actually] speak Chinese. (In other words, it lies between "I speak Chinese" and "I do not speak Chinese.")
Mona: I think that you are being modest, I am going to tell all my Chinese friends that you speak Chinese.
Raul: Don't do that, please! I really [in fact] do not speak Chinese! / Really [Believe me], I do not speak Chinese. I will be totally mortified [embarrassed] if a friend of yours speaks to me in Chinese and I am unable to answer.