it is+段时间/点时间+（when since before after until等等）的正确形式和用法分别是什么

it is+段时间/点时间+（when since before after until等等）的正确形式和用法分别是什么

ince是自从的意思
before是在~之前
一段时间 +since
表示从这个时间开始到现在---自从~以来
用法上It + ( 现在完成时 ) + 一段时间+ since + ( 一般过去时 )
例句：
1.It is / has been four days since I caught a cold.我感冒已四天了.
2.It is / has been two weeks since we met last.自从我们上次见面以来已过了两周.
一段时间 + before
表示在这个时间之前,或者大约在这个时间上就会发生某事
用法上It + ( 一般将来时 ) + 一段时间+ before +( 一般现在时 )
例句：
It will be five years before he meets you again.
五年之后他就会再见你的.
It + be (not) + long before
表示"(不)很久 / 才…"或"(没)过多久就 / …"
It was (not) long before ….(谈论过去)
It will (not) be long before ….(谈论将来)
It was long before she came back.
没过多久她就回来了.
It will be long before we meet again.
很久以后我们才会再见面.
It will not be long before we turn our hope into reality.
过不了多久我们的希望就会变成现实.
这里it指时间,before是连词,引导时间状语从句.long可用一段时间来替代.
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.香港回归已经有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） It is/has been two days since she fell ill.(她生病两天了）
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）