on earth 到底
【讲解】
这条成语常与代词who和what、关系副词 when，where, why,how等连用，表示强调，相当于后缀-ever。
【对话】
A:Why did you stand there without a word?
A:你怎么站那儿一句话也没有？
B:I didn't know what on earth to answer.
B:我不知到底该说些什么。
A:Then, why did you say sorry to the teacher?
A:那你怎么不和老师说你抱歉？
B:I don't know, I just felt afraid at that time.
B:我也不清楚自己是怎么回事，那会儿我就是觉得害怕。
A:You should change it.
A:你该把这一点改掉。
