can one say "unpolite"? As in the following sentence: "it's hard for me to be unpolite." I was in class today and my teacher asked me to give him an example of an infinite sentence and that came out of my mouth just because, but my teacher corrected me. And told me to say "impolite" instead, but I found acouple of threads on the internet with opposed ideas. So please help me with this one.
In modern English, the correct term is "impolite".
However, Google Ngrams shows that two hundred years ago, it was more common to say "unpolite".
Both in- (im-) and un- are used for different adjectives, I don't think there is a rule for which one to choose. Sometimes it changes even for adjectives and nouns that have the same root, for instance: unstable but instability.