1.在“It is+一段时间+since +从句”这一结构中，如果从句谓语动词是非延续性动词，那么时间的计算就从该动作的发生开始算起。即从句直接翻译。
It's three years since he joined the army.
他参军已经三年了。
It's a long time since they left.
他们已经离开很久了。
It’s a long time since he studied Chinese.
他好久没学汉语了。
It is ten years / It has been ten years since they last met.
自从他们上次碰面之后已经10年过去了。
It was exactly five years since her father had died.
从她父亲去世到那时正好满五年。
It is three years since he began to smoke.
It is three years since he has smoked.（这个句子欢迎大家讨论）
他吸烟已有3年了。（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，主句的动词用一般现在时、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。）
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。（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，主句的动词用一般现在时、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。）
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。
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。（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，主句的动词用一般现在时、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。）
It is two years since I became a postgraduate student.
我成为研究生已经两年了。
It is two years since I was a postgraduate student.
我研究生毕业已经两年了。
2.在“It is+一段时间+since +主语+谓语+宾语”这一结构中，如果从句谓语动词是延续性动词，时间就要从该动作的结束算起即从句就要进行否定的翻译。比如：
It's many years since they lived here.
他们不在这儿住已经好多年了。
It is three years since he smoked.
他不吸烟已有3年了。
It's three years since he was a teacher.
他不当教师已经三年了。
It is half a month since he was a League member.
他退团半个月了。
It was a long time since she had been to church.
她很久没去教堂了。
It's been two weeks now since she called.
她到现在已经有两个星期没来电话了。
It's been an age since we've seen them.
我们有很长一段时间没有见到他们了。
It's twenty years since I've seen her.
我已经二十年没见她了。
It's years since we last met.
我们多年没有见面了。
It has been quite some time since I was last in London.
我上次离开伦敦至今颇有一段时间了。
It had been 20 years since I had directed any kind of film.
我已经20年没有执导任何一部电影了。
推广与注意：
He has written to me frequently since I was ill.
自从我病愈以来，他经常给我写信。
He has written to me frequently since I fell ill.
自从我生病以来，他就经常给我写信。
I have changed my job five times since I was in Shanghai.
我离开上海后换了五次工作。
主句中主语为 it，动词用一般现在时或一般过去时，since 引导的从句中动词须用一般过去时或过去完成时。
It is two years since we last parted.
我们上次分手以来已有两年了。
It was two years since we had last parted.
那时我们已经分手两年了。