1. Since从句为非延续性动词的过去时或现在完成时，时间的起点应该从从句动作完成时刻算起。
例如：
Things have changed a lot since I wrote to you last time.
自我上次给你写信之后，情况已发生了很大的变化。
She has lived with us since she has come here.
自从她来到这里，就一直和我们住在一起。
2. Since从句为延续性动词的一般过去时，表示动作或状态的结束。其含义与动词的词义恰好相反，具有否定意味。
例如：
All has changed since he was at home.
自从他离开家以后，这里的一切都变了。
I haven’t written to her since she lived in London.
自从离开伦敦以来，我还没有给她写过信。
He has never been to see me since I was ill.
自从我病愈以来，他一直没有来看我。
Two years have passed since I last smoked.
我戒烟已经两年了。
但如果since从句是延续性动词的现在完成时，则表示动作由开始延续至说话的时候，具有肯定意味。
例如：
He has never been to see me since I have been ill.
自从我生病以来，他一直没有来看我。
She has talked little since she has stayed at home.
自从她呆在家里以后，就很少讲话。
Since we have owned a car， we have gone camping every year.
自从我们有了汽车后，年年都去野营。
3. 在现代英语中，since从句是延续性动词的一般过去时，有时候也可以表示肯定意味。此时，多半用ever来加强since的语义。
例如：
She has known me ever since she was a child.
她从小就认识我了。
I have live here ever since I was born.
我生下来就住在这里。
4. 在it is+时间+since从句结构中，since从句是非延续性动词的一般过去时，含有肯定意味。
例如：
It is three years since her husband left her.
她丈夫离开她已经3年了。
It is over sixty years since the People’s Republic of China was established.
中华人民共和国已经成立60多年了。
5. 在it is+时间+since从句结构中，since从句是延续性动词的一般过去时，含有否定的意味。
例如：
It is three years since she stayed here.
她离开这里已经3年了。
How long is it since you were a league member?
你退团有多久了?
6. 在it is+时间+since从句结构中，since从句是现在完成时或过去完成时，均含有否定意味。
例如：
It is five years since we have seen her.
我们已经5年没有见她了。
It was years since I had seen her.
在那之前，我已经有好久没有见到她了。
It’s been a long time since I’ve seen you.
我已经很久没有见到你了。
另外，①根据英语惯用法，since从句不可以否定形式出现。
例如：
It is four years since I have smoked a cigarette. 我已经4年不抽烟了。
不可说：It is four years since I haven’t smoked a cigarette.
但由于下句中的since不作“自从”解，所以从句可以用否定形式。
例如：
I haven’t been out anywhere since I don’t know how long.
也不知多久了，我哪儿也没去过。
②Since从句通常不与非延续性动词的现在完成时的肯定式连用。
例如：
Her mother died when she was a child.
她小时候母亲就去世了。
不可说：Her mother has died since she was a child.
③Since前不可加表示时间的词语，但可以与ago连用。
例如：
She has lived with us since her mother died three years ago.
自从她母亲3年前去世，她就一直和我们住在一起。
不可说：She has lived with us three years since her mother died.
④在It was five years ago since his father died. 一句中的since用错了，因为在含有ago的时间范畴里，已经有了从过去的一个时间点以来的时间，即已有了since的时间范畴，所以再用since是多余的。
改为：
It was five years ago that his father died.
他父亲5年前去世了。
It is five years since his father died.
他父亲去世已经5年了。