到底 / 究竟: on earth, after all
副词 / adverbs
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
S + 到底/究竟 + question ? | 到底 + V (= finally)
In a question, 到底 (spoken) and 究竟 (more written) show impatience or a push for a clear answer: "on earth", "exactly", "after all".
The question must be open (question word, A-not-A, A or B): never with 吗.
到底 also means "finally, in the end" (他到底成功了), and as a complement: 坚持到底 (hold out to the end).
Examples
你到底同意不同意？
Nǐ dàodǐ tóngyì bu tóngyì?So do you agree or not?
这个项目究竟什么时候能完成？
Zhège xiàngmù jiūjìng shénme shíhou néng wánchéng?When exactly will this project be finished?
经过几年的努力，他到底实现了理想。
Jīngguò jǐ nián de nǔlì, tā dàodǐ shíxiàn le lǐxiǎng.After years of effort, he finally achieved his dream.
Common mistakes
✗ 你到底同意吗？→ ✓ 你到底同意不同意？ → no 吗 with 到底.
✗ 到底你… at the start occurs in speech, but the standard slot is after the subject.