[dào dǐ]
(到尽头) to the end; to the finish
fight to the finish
打到底
hold on to [till] end
坚持到底
The experiment finally proved to be a success.
试验到底成功了。
What on earth did you say?
你到底说了些什么?
Don't get discouraged, you are new to the work after all.
别灰心, 这工作对你说来到底还是生疏的。