How soon 表示多快.这句问句的意思为：汽车多快会开动呢?
What time提问的话,回答应该是具体时间,如：At three o'clock.等.
How long 表示多久...How soon与 How long的区别是：
1.how long“多长”,询问动作在时间上所持续的长度.其答语部分或划线部分可能是：
1）“for + 时间段”,有时可能没有介词for.如：
---How long did he wait for you here?他在这儿等了你多久?
--- For two hours.两个小时.
It took him 20 minutes to finish his homework.(对话线部分提问)
→How long did it take him to finish his homework?
2）“since + 具体时间”,“since + 时间段 + ago”或者“since + 从句”.如：
---How long are you feeling like this?像这样已经多久了?
---Since last night.从昨晚到现在.
3）表示时间的between…and…,from…to…等介词短语.如：
They were playing football from 9:00 to 10:00 yesterday.(对话线部分提问)
→How long were they playing football yesterday?
另外,how long也可以询问某事物的具体长度.如：
---How long is the desk?这张桌子有多长?
---1.2 meters.1.2米.
2.how soon“多久,多快”,询问动作在将来要经过多长时间才会发生.其答语部分或划线部分可能是：in + 时间段.如：
He’s going to Zhengzhou in two days.(对话线部分提问)
→How soon is he going to Zhengzhou?