it's + a period of time + before/since 的区别?"it's + a period of time + before..." 与 “it's + a period of time since...”的区别是什么?什么时候用 since 什么时候用 before

it's + a period of time + before/since 的区别?"it's + a period of time + before..." 与 “it's + a period of time since...”的区别是什么?什么时候用 since 什么时候用 before
it's + a period of time + before/since 的区别?
"it's + a period of time + before..." 与 “it's + a period of time since...”的区别是什么?什么时候用 since 什么时候用 before

it's + a period of time + before/since 的区别?"it's + a period of time + before..." 与 “it's + a period of time since...”的区别是什么?什么时候用 since 什么时候用 before
我举个例子你就知道了
it's + a period of time + before he came 在他来之前的一段时间
it's + a period of time since he came 在他来之后的一段时间
两者的区别,这样明白了吧?