it's + a period of time + before/since 的区别?

it's + a period of time + before/since 的区别? it is some time before与it is some time since 的区别是什么,请举例说明, it's a long time 空格 we met last .a:before; b:since; 为什么? it's just a matter of time before 是什麽意思 迟早的意思吗? over a period of time Since receiving your letter over a long period of time to re A ballet dancer must____for a long period of time before she period与time的区别? time 与 period的区别 It's the time for,It's a time for,It's time for的区别 It's a long time since还是It has been a long time since 求翻译~ we believe it's only a matter of time before