it was+段时间+before结构是什么意思

it was+段时间+before结构的意思是：距离(发生某事)有多长时间，但表达的是过去的事情.
其中before是从属连词，表示“……才；……就”，引导时间状语从句。

1. It was 10 years before I left China.

我离开中国已经有10年了。

2. It was just 3 days before the book was published.

三天后那本书就出版了。

3. It was three months before he finished the work.

他花了三个月才完成这项工作.

是距离(发生某事)有多长时间的意思，但表达的是过去的事情.
其中before是从属连词，表示“……才；……就”，引导时间状语从句。
例如：1. It was 10 years before I left China.
我离开中国已经有10年了。

2. It was just 3 days before the book was published.
三天后那本书就出版了。
3. It was three months before he finished the work.
他花了三个月才完成这项工作.

it was+段时间+before
.......时间以后（发生了某件事）

如：
It was two hours before they arrived.
他们在两小时过后到达。

It was not long before it began to rain.
没过多久，就开始下雨了。

是距离.有多长时间的意思.但表达的是过去的事情.
Eg：It was 20years before i left the town.
我离开那个小镇已经有20年了

I have been away from my hometown for five years.