1.how soon 对（从即时起）到将来某个时刻间的时间长短提问,回答通常为 in+一段时间.如：
-How soon will you be back?
-In two days.
2.how long 对从某个时刻都另一时刻（各时态皆可）间的时间长短提问,但后跟的谓语动词必须为延续性动词,回答为 for+一段时间.也用作对长短提问.如：
-How long did he be away?(leave为短暂性动词,故换为短语be away)
-For ten years.
3.How often 对某个时段内谓语动词发生的频率提问,答语为 次数+一段时间（once a day, three times a week等）,也可用never、seldom等回答.如：
-How often does he visit his grandpa?
-Twice a month.