I a new to the Chinese language and something that really confused me is that in some circumstances 呢 and 吗 could be used interchangeably, but in others they cannot. For example you cannot say 你吗？ but you could say 你呢？ You can say 你是同学吗？ and 你是同学呢?. Why are the two character interchangeable in one scenario but not the other? An explanation would be great, thank you!
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