how soon指再過多久， 主要對表示將來的時間進行提問，比如：in an hour, in three weeks。how long表示多長時間，與連續性動詞連用，主要用于對一段時間進行提問，常用for+時間段回答。how often主要強調動作發生的頻率。

How long

How long多久了（問時長）,以For+時間段或since +時間點作答

How long did you stay here? I stayed here for two weeks.

How soon

How soon多久之后（問解決事情等的速度）,用in+時間段作答

How soon will your father be back? In two weeks.

How often

How often多經常（問頻率） ,常以數詞+ times a week（month 、year……）作答

（強調做了幾次）如果句子中有always, usually, often, never, once a day, twice a week, three times a year等詞時，常用how often 提問。

How often does your brother go home? Twice a year.