主要用来问频率的。比如：How often do you take exercises?你多久锻炼一次？twice a week.我一周锻炼两次.I take exercises every day，我每天都锻炼身体.
02用来提问某动作或者状态发生的频率，指的是“一段时间内发生了几次活动”。How often do your parents let you to watch TV.你的父母多久让你看一次电视？
03注意和how soon,how long的区别，how soon主要是对“多快，多久”提问？How soon will you come back？how long是就 "一段时间"提问的，how long have you lived here?
04注意和how far的区别，how far主要是就"距离"提问的，how far can you throw the ball?
