到底 = "in the end". This is used like in English as a questioning form. 他到底是谁 = In the end, who is he? In a statement like, "in the end, he was right", you CANNOT use 到底. Here you must use /終於/最後/ 他是对的. Other meanings & usage: (2) [at last]∶表示经过较长过程最后出现某种结果 经过一番曲折,事情到底成功了 (3) [after all]∶强调原因或特点;毕竟 [南方] 到底是南方,四月就插秧了 [小孩] 到底是小孩,这些道理他还不大懂 你到底为什么不走进那间屋子 (4) [through;to the end]∶一直到完毕、结束或完成 贯彻到底 (5) [after all]∶表示感叹的语气 到底还是女人心细 http://www.zdic.net/c/0/11/28443.htm

他是谁 Who is he ? 他到底是谁 Who is this f**king man ? 你到底为什么还没来？ I am angry . 你为什么还没来？ I still hava some patience. lol...23333333333

We usually use 到底 sometimes when you can't get the precise answers from the person you are asking.For example, if you ask your partner 他是谁？Then he said uhm.... he is new here,he is like 25years old belabela:p..Then you may need to say 他到底是谁？Personally think,it's a bit like Who is he exactly...So,emotionally,when people have a strong desire to know about something,a stranger's name, reasons or any other informations,and wanna get more precise feedback. However,I would regard it as a little bit inpolite word,so I wouldn't use it often in formal occasions or even in my daily life.Because when use 到底 that means I have already run out of my patience listening to your stupid response,and I'm a little angry now and I need to know your exact answer immediately:) Not professional enough,but hope these words can help you to understand.Thank you.

他到底是谁? Who on earth is he? Or who the hell is he? 他是谁? Who is he? 你到底为什么还没来? Why on earth haven't you come? 他是谁?Why haven't you come? In this case, 到底 is used as a modal adverb to emphasis one's tone, usually to express angry, anxiety and/or curious. PS: when you see 到底 in sentences or phrases, sometimes it means 'to the end' where 到is individually used as preposition "to". eg. 坚持到底 means "stick to the end". 走到底means "go straight to the end'.

到底=究竟 jiu1 jing4