-(이)야 is a casual "이에요/예요" which is for nouns. For example you would say "전 미국 사람이에요" in 존댓말, but you can say "나 미국 사람이야" to your close friends. I should point this out, 아니요 is not a polite version of 아니야. Casual Korean : 아니. No. Honorific Korean : 아니요. Casual Korean : 아니야. It's not. / I'm not. / s/he's not. / They're not Honorific Korean : 아니에요.

I wonder if you know that there are several speech levels in Korean.^^ Ending "요" as in "아니요" indicates "해요체(Haeyoche)" which is informal and polite and is often used in conversation. This speech style is used for strangers or elderly persons than you to show them respect. Whereas "아니야" is "해체", which is informal and impolite, and commonly used when talking to close friends or younger persons than you. Here are some other speech styles for "아니요" (All are the same in meaning) 아닙니다. - 합쇼체(=하십시오체) Hapsyoche 아니오 - 하오체 Haoche 아니요 - 해요체 Haeyoche 아니네 - 하게체 Hageche 아니다 - 해라체 Haerache 아니야(=아냐) - 해체 Haeche Here "체(體 - che)" means "a style". As for different speech styles, you can get more information at http://en.wikipedia.org/wiki/Korean_honorifics ^_______^