难道 vs. 莫非

难道=难不成

莫非=莫不是

“难道”比“莫非”更为常用。
难道他不来了？
莫非他不来了？

难道不是你写的信？
（“莫非”无法在这个例句里进行替换，会显得晦涩）

在口语中，“难道”更为常用。

 难道：副词。加强反问的语气：你能学好外语，难道我就不能？

难道 seems a bit more informal, more common and it also seems to strengthen the rhetorical nature of the sentence, therefore a response is not necessary.

莫非：副词。表示揣测或反问：莫非我看错了人？

莫非 seems a bit more formal, less common and although also used most often in rhetorical questions, it may be more likely to receive a response.