①It was three years since Jack had been a teacher.杰克不当老师有三年了。（指过去）

②It is three years since Jack was a teacher.杰克不当老师有三年了。（指现在）

注：句②中的was可以用has been，但意义不变。

▲请看英语语法网上的两篇文章：

http://www.yygrammar.com/Article/201501/3791.html

http://www.yygrammar.com/Article/201504/3916.html

   【答】这两句有什么的意思不同。具体译文见下面的解答。第二句的was也可以改为has been。

   “It was + 段时间 + since ... 延续性动词的过去完成时...”表示从某一过去动作或状态结束时起到另一过去时刻为止已经有多久了。例如：It was three months since we had been here. 那时我们已经有3个月没来这儿了。（摘自《薄冰英语惯用法词典》）

所以：It was three years since Jack had been a teacher. （到）那时我已经有3年没有当老师了。

    “It is + 段时间 + since  ...延续性动词的现在完成时或一般过去时 ...”表示从某一过去动作或状态结束时起到现在（说话时刻）为止已经有多久了。例如：It is two months since Tom has smoked a cigarette.(= It is two months since Tom smoked a cigarette.) 汤姆已有2个月不抽香烟了。（摘自《薄冰英语惯用法词典》）

    所以，It is three years since Jack has been a teacher. =It is three years 

since Jack was a teacher. （到现在）杰克已经有三年不当老师了。【或】（到现在）杰克已经离开教育战线三年了。

赞同曹老师解答观点

(1)It is +时间段+since +主语+持续性动词过去式（表示动作结束算起到现在有多长时间）

It is five years since he lived here.

他没有住这里有5年时间了。

(2)It is +时间段+since +主语+瞬间性性动词过去式（表示动作开始算起到现在有多长时间）

It is two years since he came here.

他来这里有2年时间了。

(3)It is +时间段+since +主语+持续性动词完成式（表示动作开始算起到现在有多长时间）

It is two years since he has been ill.

他生病有2年时间了。