how long表示多长时间，与连续性动词连用，主要用于对一段时间进行提问，常用for+时间段回答。
例如：How long did you stay here? I stayed here for two weeks.
how long 还可以表示东西有多长
例如：How long is the river? About 40km.
How often主要强调动作发生的频率。（做了几次）如果句子中有always, usually, often, never, once a day, twice a week, three times a year等词时，常用how often 提问。
例如：How often does your brother go home? Twice a year.
how soon指再过多久， 主要对表示将来的时间进行提问，比如：in an hour, in three weeks
例如：How soon will your father be back? In two weeks.
how far 意为：多远，对距离进行提问
例如：How far is it from your home to school? About four kilometers.
加强训练
共同选项：A. How far B. How long C. How soon D. How often
(1) —______ will you come back? —In half an hour.
(2) —______ can I keep the book? —For only a week.
(3) —______ do you visit your parents? —Once a month.
(4) —______ is the road? —Three kilometers.
(5) —______ is the road from your school? —Three kilometers. / Ten minutes’ walk.
(6) —Do you know ______ we will take part in the High School Entrance Exam?
—Look at the blackboard. There are only forty days left.
答案：CBDBAC