更正楼上答案如下:
1.How soon will her parents come back from the USA?
2.How often do you write to your penfriend?
3.Why did he buy some meat?
4.How long did it take them to finish the work?
5.What does Tom often do in summer?
6.How much time ago was the old stone bridge built?
如果是对two hundred years ago划线提问,则是:When was the old stone bridge built?
如果是对two hundred划线提问,则是:How many years ago was the old stone bridge built?