用法1：
since引导的时间状语从句中用短暂性动词的过去式，主句用现在完成时或现在完成进行时。此时，整个句子可以直接按照字面意思翻译。 例句
She has dealt with five guys since she divorced Tom.
她和汤姆离婚后和五个男人交往过。
I have been missing her since she left.
自从她离开我就一直很想念她。
用法2：
since引导的时间状语从句中用延续性动词或者状态动词的过去式，主句用现在完成时或者现在完成进行时。此时，时间状语从句指示的时间是这个延续性动作结束直到现在这个时间段，翻译时不能按照字面翻译。 例句
It has been almost five years since I taught high school students.
我不教高中已经大概有五年了。
I have changed my job five times since I was in Shanghai.
我离开上海后换了五次工作。
用法3：
since引导的时间状语从句中的动词用延续性动词的现在完成时，主句的动词用现在完成时或者现在完成进行时。此时，状语从句指示的时间是延续性动作开始一直到现在这个时间段，翻译时可以直接按照字面意思理解。 例句
It has been almost five years since I have taught senior three students.
我从教高三开始到现在已经五年了。
I have never heard from her since she has worked there.
自从她去那工作到现在为止，我没有收到过她的来信。
用法4：
since引导的时间状语从句中的动词用现在完成进行时，主句谓语动词用一般现在时。此时，不管since引导的时间状语从句的动词是延续性的还是非延续性的都不影响句子的意思，按照字面直接翻译即可。 例句：
We think highly of your up-grading spirit since you have been trying to change yourself. 从你一直努力改变自己以来，我们都高度赞赏你的上进精神。
We all admire you since you have been doing everything to help the poor.
自从你一直做任何事情帮助穷人以来，我们都很钦佩你。
小结：只有在since引导的时间状语从句中用延续性动词或者状态动词的过去式，主句用现在完成时或者现在完成进行时。这种情况下，时间状语从句指示的时间是这个延续性动作结束直到现在这个时间段，翻译时不能按照字面翻译。（我的意见是按照否定意味来翻译这个世界终于从句即可）。另外，词典里出现了一些it例句可以总结出：
It's 一段时间 since 一般过去时；
It's 一段时间 since 现在完成时；
It was 一段时间 since 过去完成时；
It had been 一段时间 过去完成时；
这两个句型的翻译也是否定从句含义来翻译：
如：
It's only a year since I last saw the child and he's grown so tall.
这孩子一年不见，长这么高了。
《汉英大词典》
It's twenty years since I've seen her.
我已经二十年没见她了。《牛津词典》
How many years is it since we've seen each other? Too many, anyway.
我们多少年没有见面了？反正有年头了。
《柯林斯高阶英汉双解学习词典》
It was some weeks now since they had had any respite from shellfire.
他们已经好几周没有在炮火中获得任何喘息的机会了。
《柯林斯高阶英汉双解学习词典》
It was a long time since she had been to church .
她很久没去教堂了。《柯林斯高阶英汉双解学习词典》
It had been two years since I had been to the swimming baths.
我已经有两年没去过室内游泳馆了。
《柯林斯高阶英汉双解学习词典》
其他情况都可以正常用法翻译。
