how soon, how long, how often

虽然 how soon, how long, how often三者均可译为“多久”，但很明显它们表达的意思是不一样的。

1. 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 等的提问。如：　　

A: How soon will he be back?

B: In an hour.

　

2. how long 指多长时间，主要用来对一段时间，如：three days, four weeks 等的提问。如：　　

A: How long did he stay here?

B: About two hours.

3. how often 指每隔多久，主要用来对频度副词或状语，如：once a week, three times a month 等的提问。如：　　

A: How often does he come here?

B: Once a month.