않냐? is a colloquial expression, derived from 않니? 얂냐 is a lot less informal than 않니, and often used between very close friends. Or, an older person can use it when he/she talks to a younger person. However, if you don't use it in the right situation, you can sound very rude. i.e. Why don't you love her? 왜 사랑하지 않냐? The flower is beautiful, isn't it? 꽃이 아름답지. 그렇지 않냐? Isn't that so? 그렇지 않냐?