It+be+……+that/since/before/when用法辨析
1. it was+ (时刻，或时间点）+ when sth. happened. 当某事发生时，是几点钟或哪一年如：
It was 1997 when Hongkong returned to China.
It was five o'clock when we arrived at the small mountain village.
对比强调句型：It was at five o'clock that we arrived at the small mountain village.
It+ is/ was+…+that…多用于强调。
判断方法如下：去掉it、be和that，余下的部分仍是一个完整的句子，这样的句子一定是强调句。
2. it was+一段时间+ before sth happened. 过了一段时间之后，某事才发生了。如：
it was two years before Macao returned to China.
it will be +一段时间 before sth. happens. 要过一段时间之后，某事才会发生。
或不用过多久，某些就将发生：it won't be long before sth. happens.
It won’t be long before you see him again.
没过多久就…
It was long before they escaped from the prison.
过了很久才….
3. it is (=has been)+一段时间since sth. happened. 自某事发生以来，（时间）已经过去。。。了。
如：It is 11 years since Hongkong returned to China. 回归已经有11年了。
若从句中的动词是终止性动词，意为：“从开始……多久了”例如：
It has been five years since she left home.（她离家已经十五年了）
It was four years since he had died.（到当时为止，他已死去四年了）
若从句中的动词是延续性动词，意为：“不做某事已有多久了”例如：
It is two days since she was ill.（她病好了两天）
It is two years since he studied English.（他已有两年没学英语了）
练习：请用before，since，that，when填空。
1.It was last Sunday _____ I came across Mr Brown.
2.It was midnight _____ the young couple came back from the ball.
3.It won’t be a minute _____the lunch is ready.
4.It is two weeks _____the Smiths lived in our village.
5.It was not until 1920 _____ regular radio broadcasts began.
6.Was it during the Second World War _____ he died?
7.It has been one year _____ I got the job.
8.It wasn’t long _____ they drove the enemy off their country.
9.It will be two years _____ we meet again.
10.It is years _____ I enjoyed myself so much.
