how long?how soon ?how often?
what's the difference between them?
if someone asks me those questions,how should i answer him?
and how to use them when i ask others

Question

how long?how soon ?how often?
what's the difference between them?
if someone asks me those questions,how should i answer him?
and how to use them when i ask others

Answer

how long - 
usually refers to length, but can refer to the amount of time you have to wait.
Example, "How long is a that rope ?", "How long do I have to wait to see the doctor ?"

how soon - 
refers to the time until some event. 
Example. How soon before dinner is ready ?

How often -
refers to the frequency with which an event occurs.
"How often do I need to take the medicine ?" Answer: " 3 times each day, with meals !" ( 3 times per day)
"How often do you come here?" Answer: "Every Tuesday!" (once per week)

A.

Answer

These are three very different words. I am sure the dictionary will give you the answers you need.