难不成【nánbùchéng】

难不成 voices a suspicion: "could it be that…?", "don't tell me…". It is spoken and lively. It stands at the start of the clause or right after the subject, and the sentence often ends in 吗 or in a second 不成.

A guess

Most often the speaker sees something odd and floats an explanation they half believe.

Disbelief

With a 连…都 or a demand, the guess becomes a protest: the speaker can't believe it and expects the answer "no".

难道 does the same work in both speech and writing. 难不成 is the more colloquial of the two and leans toward the guess rather than the protest.

Not a plain question

难不成 always carries suspicion or surprise. A neutral question with it sounds like an accusation.

• × 难不成你吃饭了？ (just asking) → 你吃饭了吗？