吗 or 呢

Both particles end questions, but they do different jobs.

吗 (ma) turns a statement into a yes/no question: 你是中国人 → 你是中国人吗？ (Are you Chinese?)

呢 (ne) is used where 吗 is impossible:

• 'And you? / What about…?' after a noun or pronoun: 我是美国人，你呢？
• 'Where is…?' after a bare noun: 我的钥匙呢？ (Where are my keys?)
• 'What if…?': 如果明天下雨呢？
• questions that are already questions, either 'A-not-A' (来不来, 有没有) or with a question word (什么, 怎么, 什么时候). 吗 can never be added to them; 呢 makes them softer.

Rule of thumb: if the sentence already contains a question word or an 'X not X' pair, or it is only a noun, never use 吗.