辨析how soon,how often与how long
一、how soon表示“要过多久以后,要到什么时候”的意思.用来对句中带有in no time,immediately,before long,（very）soon,in five days（weeks,months,years,etc．）,after Monday（January,1949 etc．）提问.如：
He’ll come back in two hours.→How soon will he come back?
二、how often表示“多长时间一次,是否经常”的意义,通常用来对sometimes,usually,never,seldom,often,always,every day,every two weeks等表示频度的副词或词组提问.如：
He writes to his mother once a month．→How often does he write to his mother?
三、how long表示“多长时间”的意义,用来对句中带有for,unit（till）,since,all the morning,from…to…等表示一段时间的状语提问.对某些动词（take,spend等）后面表示时段的名词提问时也用how long.如：
Mr.Black has lived in Shanghai for over two years．
→How long has Mr.Black lived in Shanghai?
