For many expressions in Japanese, there are pairs for different situations. What you've mentioned here is one of the many pairs. "jya nai" is used in more casually, e.g. between good friends, and also used by someone of a higher social status to someone of a lower status, e.g. used by a boss to his employees, etc. "jya arimasen", on the contrary, is a more polite way of expressing the same meaning as "jya nai". In fact, it is a matter between "nai" and "arimasen". Both of them are the negative form of the verb "aru", with "nai" being the casual one and "arimasen" being the polite one. One more thing related to this question: actually the "most" polite and formal way of expressing the same meaning is "dewa arimasen". "jya" is the shortened form of "dewa", and is more often used in oral Japanese. And "jya nai" has a even shorter from: "jya nee", which is often heard in anime and TV series, used by men informally and very casually or even rudely. It's not a simple matter of rudeness, but it depending on the situation and who you are talking to. It doesn't matter at all if you are chatting with a familiar friend if you use "jya nai". However, with a stranger, an elder, or anyone of higher social status than you, it'll be considered impolite. jya nai = じゃない jya arimasen = じゃありません aru = ある dewa = では jya nee = じゃねぇ