区别主要是
how soon是对将来时间提问。如:He will be back in two days。如果就in two days提问,要用how soon。而how long是对一段时间提问。如:He usually spend two hours to do his homework every day。如果就two hours提问,要用how long。
how soon
多久;多快;多久以后;
[例句]Can you tell me how soon you can be ready?
你能告诉我你多久能准备好吗?
how long
conj.多长时间;
[例句]How long have you been employed at this job?
你受雇做这个工作有多久了?