I’m confused. Which one should I use in Japan if I want to be polite? -nai desu, ja arimasen or de wa arimasen?
on Youtube I’ve learned to use -nai desu, for example “nihonjin janai desu” or “samukunai desu”. (in short, I just have to add ‘desu’ to a non-keigo form to make it polite, if it’s an “I” adjective) They said that ‘de wa arimasen’ sounds too cold and unnatural and I should use it only to be super polite. But the other video said that ‘-nai desu and ‘ja arimasen’ can be only used with close friends, sounds informal and casual and I should use ‘de wa arimasen’.
Now, I don’t know which one I should use. Let’s say, I’m going to Japan. I’m talking to a stranger e.g a cashier, server or meeting somebody on the internet. Would it be rude to say for example “日本人じゃないです’?