B1
到底 and 难道 — pressing, and disbelieving
How Chinese sharpens a question with 到底 and turns one rhetorical with 难道, and why 到底 cannot be used with a 吗 question.
到底 — what, exactly
到底 presses for a definite answer, usually after not getting one. English reaches for on earth, exactly, or in the end.
你到底想要什么？
Nǐ dàodǐ xiǎng yào shénme?
What exactly do you want?
他到底来不来？
Tā dàodǐ lái bu lái?
Is he coming or not?
It sits before the verb, or before the question word if there is one. 究竟 is the formal equivalent and is common in writing.
It cannot go with 吗
This is the rule worth knowing, and it follows from what 到底 is for. It demands a definite answer, so it needs a question that lays the alternatives out — a question word or an A-not-A. A 吗 question is too open to press on.
你到底去吗？
你到底去不去？
吗 invites a yes or a no and leaves the door open. 到底 is trying to close it, and the two work against each other.
难道 — surely not
难道 is not really a question. It states something the speaker finds hard to believe and invites you to deny it. The sentence usually keeps 吗 at the end, which is the one place 吗 is not doing its ordinary job.
难道你不知道吗？
Nándào nǐ bù zhīdào ma?
Surely you knew?
难道我错了吗？
Nándào wǒ cuò le ma?
Don’t tell me I was wrong?
The expected answer is no, of course not — which is why 难道 so often carries reproach. 难道你忘了吗 is not asking whether you forgot. It is telling you that you did.
Together
They can appear in one sentence, and the effect is exactly as impatient as it sounds.
你到底去不去？难道还要我求你吗？
Nǐ dàodǐ qù bu qù? Nándào hái yào wǒ qiú nǐ ma?
Are you going or not? Do I have to beg you?
Both are strong. 到底 is fine with friends and brusque with strangers; 难道 is close to an accusation, and worth recognising long before you use it.