how soon是用于將來時(shí)態(tài),意思為什么時(shí)候.但具體也看語境.
比如：how soon will you come back?
how long的時(shí)態(tài)一般用于過去時(shí),也看語境吧.
e.g.How long have you been here?
注意,how long是一個(gè)時(shí)間段而不是時(shí)間點(diǎn).
How often很容易解釋啦.它是一個(gè)對(duì)頻率提出問題的疑問詞.
比如：How often do you watch TV every week?
回答這句話的話,應(yīng)該使用頻率.比如：three times.