还没: Have you had lunch? no, not yet. 你吃午饭了吗？没(不)，还没(呢)。As Chinese, we don't need to tell "no" and "not", all of two words have the same meaning"没，没有". "还" "呢"is just to emphasis "没"。maybe I can help you.

還沒,means NOT YET,STILL NOT, 沒, means NO,NOT 了,呢,modal particle

还没：not yet. Used in perfect tense to express sth is not done. 了：particle or an auxiliary word to assist perfect tense. 没：negative word to deny perfect tense. 呢：particle often used in spoken Chinese.