It was quite a few years before I quitted the job.
此句before若改为since 则was要改成is ,为什么?
三个句型：
1.It was +一段时间+before sth.happened.
此句必有前文,也是一句型：
2.It was +时间点 +when sth.happened.当A事情发生时,时间是XXXX
如：It was 1997 when HK returned to China.
接下来：It was two years before Macao returned to China,too.
HK回归之后,MC回归之前,这一段时间是2 years.
常译作：过了两年,MC也回归了.如果是not long (before...)则译作：不久MC也回归了.
第三个句型是：
3.It is +一段时间+since sth.happened.
此句简化自：It has been 一段时间 +since sth.happened.
这你是理解的,自从某事发生以来,到现在为止已经有一段时间了.since的从句为一般过去时,与它相应的主句用现在完成时.
所以原句译文：过了好几年,我才辞职不干了.
改后译文：自从我辞职以来已经有好几个年头了.
再给你介绍第四个句型：
此句型与第二个句型相关：
4.It was 介词+时间点 +that sth.happened.
引句型比句型2在时间点前多加了一个介词,则成为强调时间状语的强调句型,如：
It was in 1997 that HK returned to China.
为HK returned to China in 1997之强调时间状语 in 1997.
请看两句的细很差别：
It was in 1997 that HK returned to China.
It was 1997 when HK returned to China.
注意句型1还有一个将来(包括情态动词）的变体：
5.It will be some time before sth.happens.
要过多久某事才会发生.
1.5句型根据时间的长短（相对的)有两种不同译法：
a.短：没过多久,某事就发生了（就将发生）
b.长：过了很久,某事才发生.（要过很久,某事才会发生）
PS：不懂从句连词者,1、5句型常误用after,皆因受汉译干扰之故：
二年之后MC也回归了：
* It was two years after MC returned to China,too.
将It was two years after 看成一个意群.