We have known each other since I was at college.

We have known each other since we were at college.

请问这两句话since从句的起算时间一样吗？从什么时候算起？

解答如下：

引导时间状语从句有以下几种情况

【1】since +瞬间动词的一般过去式/现在完成时，其动作起点以从句动作开始时间算起。(具有肯定含义）

A:从句用一般过去式

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。

Great changes have taken place since you left.

自从你离开后发生了很大变化。

B:从句用现在完成时

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。

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。

【2】since +持续性动词的一般过去式，其动作起点以从句动作结束时间算起。（具有否定含义）

He has written to me frequently since he was ill  

自从病好以来，他经常给我来信

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.

自从我病好了，他从来没有看我。

All has changed since he was at home.

自从他离开家以后，这里的一切都变了。

【3】从句谓语为持续动词/瞬间动词的现在完成式，其动作起点以从句动作开始时间算起。

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。

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。

Considerable time has elapsed since we have been here.

只从我们来这里之后，过去了很长时间了。

Since I’ve been at this school, we have had three headmasters.

自从我到这所学校，我们已换了三个校长了。

【4】since 从去前有ever加强其语义，从句为持续性动词一般过去式，其动作起点以从句动作开始时间算起，表示肯定含义。

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。

He has lived here ever since he was five years old.

自从他5岁就住在这里。

He has been in low spirits ever since his father stayed here.

自从他父亲在这里以来，他的情绪一直都比较低落。