难道
Meaning
难道 is an adverb that strengthens a rhetorical question. The speaker is not really asking — they already believe they know the answer and are expressing surprise. 吗 usually closes the sentence, but it can be left out.
Basic structure
难道 + Clause + 吗？
难道 + Clause？
The two key cases
1. A positive rhetorical question → the meaning is NEGATIVE. 2. A rhetorical question with 不 / 没 → the meaning is POSITIVE.
难道汉语真的这么难吗？
难道你不减肥了？
Examples
怎么又买了这么多饼干和巧克力？
难道你不减肥了？
喂，都十点了，你怎么还不来？
难道你没看短信？我今天有事去不了了
你不是两点就出发了吗？难道路上花了两个多小时？
我那辆车出了点儿问题，刚才把车送去修了
难道 vs. a plain 吗 question
你不减肥了吗？
难道你不减肥了吗？
More examples
难道你忘了？
难道他不知道？
A plain question vs. one with 难道
你不去吗？
难道你不去吗？
难道 turns a plain question into a strong rhetorical one.
The two forms side by side
难道你不喜欢吗？
难道你喜欢吗？
Remember:
难道 = surely…?, don't tell me…
It strengthens a rhetorical question and expects no answer.
Positive form → negative meaning; negative form → positive meaning.