how long, how often, how soon

　　how long指多长时间,主要用来对一段时间(如three days, four weeks 等)提问?如:How long ago was it? 这是多久前的事了?

　　how often指每隔多久,主要用来对频率副词或状语(如once a week等)提问?如:—How often does he come here? —Once a month. 他(每隔)多久来一次?每月一次。

　　how soon指再过多久,主要用来对表示将来的一段时间(in an hour, in two weeks 等)提问?如:How soon can you come? 你多快能赶来?