adverb, verb, noun · used in questions for emphasis to press for a definitive answer or to get to the heart of the matter

你到底去不去？

verb · through to the very finish or down to the bottom

我们一定要把这项工作干到底。

verb, adverb · at last or in the end after a process or a period of time

经过不断的努力和尝试，我们到底还是解决了难题。