不是 不是 = isn't isn't 不不不不 = no no no no

if the question contains 是不是, then use 不是 to say no. Examples that might help you understand it. 是不是你偷我的面包？ was it you who stole my bread? 不是。 我们想把面包给你吃。we want you to eat this bread. 不不不不。showing disapproval of what has just been said or a way to show your humbleness. In a movie, you see someone steals your last piece of bread, you go: 不！！！！！ or 不要！！！！ as you can tell, i am hungry.

不是 means am not/ isn't/ aren't/ wasn't/ weren't. 不 can't stand on its own. it must be added with verb-to-be/ aux. verb or verbs themselves. (不+verb-to-be/aux. verb or 不+verb).