1. how long

指多长时间，主要用来对一段时间(如three days, four weeks 等)提问：

How long ago was it? 这是多久前的事了?

How long do you think you’ll be gone? 你想你会离开多久?

How long have you known her? 你认识她有多久了?

How long have you been studying the problem? 这问题你研究多久了？

“How long did he stay here?” “About two hours.” “他在这儿呆了多久?”“大约两小时。”

2. how often

指每隔多久，主要用来对频度副词或状语(如once a week等)提问：

How often does he come here? —Once a month.

他(每隔)多久来一次? —一每月一次。

How often do you visit your mother?一Once a week.

你多长时间看你妈妈一次?——一周一次。

How often do you go to the dentist?一I go every six months.

你多久去看一次牙齿?——我每6个月去一次。

3. how soon

指再过多久，主要用来对表示将来的一段时间(如in an hour, in two weeks 等)提问：

How soon can you come? 你多快能赶来?

How soon shall we know the results? 一I don’t know. 我们多久能知道结果? —我不知道。

How soon will he be back?—In an hour. 他要多久才回来?—1小时以后。

How soon will you be / have finished, dear? 亲爱的，你还要多久才能准备好呢?

How soon will you know your holiday dates? 你什么时候才能知道休假13期?