1. It be +时间段+since表示自从以来有多久了，但若从句谓语是延续性动词时，则意为(多久) 不做了。
2. It be+时间段+before表示过之后才，若be是否定式，则表示没过就。
3. It be +时间点+before表示在之前，时间是。
4. It be +时间点+when表示当时，时间是。
It is three years since I joined the Party. 我入党有三年了。
It is three years since he lived here. 他不住这里有三年了。
It will be years before we meet again. 要过几年之后我们才能重相聚。
It was not long before he came back. 没过多久他就回来了。
It was evening before they got here. 他们到达这里之前就已天夜了。
It was five oclock when he got home. 他回到家时已是五点了。(比较强调结构：It was at five oclock that he got home. )
