到底

到底 (dàodǐ) has two main uses. As an adverb in questions it intensifies and demands a definitive answer — roughly English 'on earth' or 'exactly': 你到底想要什么？('What on earth do you want?'). Here it presses the listener to stop dodging and commit. As an adverb in statements it means 'after all,' 'in the end,' or 'when all is said and done': 他到底还是来了 ('In the end he came after all').

Literally 到底 means 'reach the bottom,' and it can also be used physically — 坚持到底 ('persist to the very end'). English speakers should note the interrogative use has no clean single-word equivalent; it adds impatience or insistence to a question. 到底 is neutral in register and very common in speech. In a question it cannot combine with 吗 — you use it with a question word (什么, 谁, 为什么) or an A-not-A form.