it was 时间when it was 时间before it was 时间since 这几个句型有什么区别

it was 是指某事物过去........
when it was是当它......时（过去时）
before it was 是指之前（过去）它.......或在它....之前
since+过去时间点指从......（时间）到.......

It was/will be+时间段+before 过了…才… eg:It was three years before he finished the work.他花了三年才完成这项工作. It will be three

四个句型五个例句：
句型1.it was 时间点 when sth.happned.
句意：当某事发生时,时间是XXX.
例：It was 1997 when Hongkong returned to China.
主句：it was 时间点.it 是主语（不是形式主语,就是主句的主语）,指代时间.（常见于：it is six o'clock now.It was ten o'clock when I got home.) when-从句为 时间状语从句,也可以放在主句前面.When sth.happened,it was XXX.
句型2.it was 时间段 before sth.happened.
意为：（在某件事情发生之后）,另一件事情发生这前,这段时间是.
翻译1.过了.,就发生了另一件事.(主观认为时间较短）
翻译2.过了.,才发生另一件事.（主观认为时间较长）
例：It was 2 years before Macow returned to China too.过了两年澳门也回归中国了.
析：这句话前有例句子1为前提,是1997年之后,1999年之前这一段时间的长度(it was two years.)
误：It was two years after .,两年后,.也.了.
补充：before的前提是：前面发生过一件事,从这个时间开始,到另一件事情发生之前(before) 这一段的时间（时间段）是多长.
如果这个时间段长或不长,有两种不同的译法.
1.才过了多久就.了.
其中最常见的一个例句就是：It wasn't long before sth.happened.不久就.了.
2.过了很久才.
时间的长短是一相对的概念.
二年时间相对百年的殖民时代是短的,所以可以译作：才过了两年澳门也回归了.
但如有人考了两年才考出驾照.
He began to learn how to drive a car in 2005.
（It was 2005 when he began to learn how two drive a car.)
It was two years before he got the drive licence.就得译作：过了两年才领到驾照.
句型3.it is (=has been) 时间段 since sth.happened.
意为：自从某事发生以来（到现在为止）时间已经是多久了.
It is (= has been ) 14 years since Hongkong returned to China.香港回归中国已经有14年了.注：本来就是（现在完成时）has been 与since 从句连用.现代英语用is代has been了.
4.强调句型：It was in 1997 that Hongkong returned to China.香港是在1997年回归中国的.
注：before 句型的另一个时间,将来时态：
It will be N years before TW returns to China.( before TW is liberated.)
数轴表达示意：
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---------+---------+---------------------------------->.> Time
1997 1999 now future