It be +时间段+before... It be+ 时间点 +when... It has been +时间段 +since...分别是什么意思及它们的用法?
It is three times years since they got married.与It is three times years since they be married在意思上有什么区别?It was three years before he came back form abord.是什么意思? It won't be long before we meet again.是什么意思?