1. It be +時間段+since表示自從以來有多久了，但若從句謂語是延續性動詞時，則意為(多久) 不做了。
2. It be+時間段+before表示過之後才，若be是否定式，則表示沒過就。
3. It be +時間點+before表示在之前，時間是。
4. It be +時間點+when表示當時，時間是。
It is three years since I joined the Party. 我入黨有三年了。
It is three years since he lived here. 他不住這裡有三年了。
It will be years before we meet again. 要過幾年之後我們才能重相聚。
It was not long before he came back. 沒過多久他就回來了。
It was evening before they got here. 他們到達這裡之前就已天夜了。
It was five oclock when he got home. 他回到家時已是五點了。(比較強調結構：It was at five oclock that he got home. )
