how long 与 how soon 都可译作“多久”，它们有区别吗？又，它们与 how often 有什么不同？
how long 用于询问时间的延续长度。在这种问句里要使用持续性动词，可用现在时、将来时、过去时等。在答句里要使用表示时段的词语，例如，for two weeks, since 1999:
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How long will he stay here? 他将在这里呆多久？
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How long is the speech? 发言有多长?
how soon 用于询问某个事件要在多久以后发生。
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——How soon will they come back? ——In two days. ——他们多久回来？——两天后。
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How soon can you be ready? 你多久能准备好？
how often 是询问事件发生的频率, 意为“多久一次”:
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“How often do you go there?” “Once a month.” “你多长时间去那里一次?”“每月一次。”
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How often do you visit her? 你多长时间去探望她一次？
请做下面的英语试题：
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The grand hotel has been completed, but we don't know ____ come into use. A. how long it will B. how long will it C. how soon will it D. how soon it will(上海会考,94-15)
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He asked me ____ stay. A. how long I was going to B. how long was I going to C. how long time I was going to D. how long time was I going to
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“How often did you lend him money?” “____ than I can remember.” A. The more time B. More time C. More times D. The more times
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——How long ____ at this job? ——Since 1990. A. were you employed B. have you been employed C. had you been employed D. will you be employed(北京高考,03 春 -27)