verb · used to ask for confirmation or to form a direct question about whether something is the case

你是不是还没吃饭？

verb · an interrogative phrase used to ask for confirmation or to form a yes-no question, functioning similarly to an auxiliary for asking if something is the case.

你是不是没吃饭？

verb · used to introduce an indirect question or a clause expressing uncertainty between two possibilities

我不知道他是不是已经回家了。

verb · an interrogative phrase used to ask whether something is the case or to seek confirmation.

他是不是还没来？