I hope a Korean native will answer, because I m not! Sorry for that, but, to the best of my knowledge, the only correct way to say: "That s not a tree" is 저거 나무(가) 아니에요 (beware of the spelling, u can also commonly find 아니예요). This is impossible: 저거 나무지 않아요! Now, for me, the only possible interpretation of 저거 나무잖아요! is positive, something like "That's a tree, don't you see?". There s a flavour of "contrary to what u seem to think, that's a tree!" Would appreciate help from a native though...

Hujok is correct^^ you are 한글 is good. 저거 나무 잖아요! --- That's a tree That's not a tree! --> i will rewrite --> 저건 나무가 아냐( or 아니에요) 2. 저거 나무 안예요 and 저거 나무 안 이예요 - both are impossible sentance,