since的用法主要有两点：一表示“自从……以来”，二是表示“既然”之意。但如果只了解词的含义，而不了解其用法，在使用时就会造成错误。例如：I have not heard from him since I lived in Shanghai. 不少人会望文生义，将该句译为：“自从我住在上海以来，就再也没有收到他的信了。”而实际上这个句子的译文并非如此，那么正确的含义是什么呢？下面谈谈since的用法。

一、若since引导的状语从句的谓语动词是终止性的过去时，则从句表示的时间是“从动作开始的那一时刻起”。如：

1）He has studied very hard since he came to our school.

自从他来到我们学校，他学习就非常努力。

2）We have been missing them since they left here

自从他们离开这里，我们就一直很想念他们。

这是最常见的一种用法，此处不再多述。

二、若since引导的状语从句的谓语动词是持续性动词或表示状态的动词的过去时时，则从句表示的时间是“从那持续动作或状态结束时算起”。如：

1）I haven’t heard any noise since I slept.

Sleep 为持续性动词，sleep的动作结束时，即“醒来”时，这句应译为“我醒后还未听到任何声音”。

2）John is now with his parents in New York, it is already three years since he was a teacher

约翰现在和父母一起住在纽约，他不当教师已经三年了。

本文开始出现的那个句子就可以译为“自从我离开上海，就再也没有收到他的信。”这种译法常被人们误用，我们不妨再多举几个例子看看：

3）How long is it since you lived in Shanghai?

你离开（没住在）上海多久了？

4）It is half a month since he was a League member.

他退团（不当团员）半个月了。

 5）It’s been quite some time since I was last in London.

我离开伦敦已很长时间了。

比较：He has written to me frequently since I was ill.

自从我病愈以来，他屡次给我写信。（从句谓语为状态动词）

He has written to me frequently since I fell ill.自从我生病以来，他就屡次给我写信。（从句谓语为终止性动词）

三、若since引导的状语从句的谓语为持续性动词的现在完成时，则表示动作和状态延续到现在（说话时刻），其表示的时间的起点应从动作发生之时算起。如：

1）I haven’t heard from him since he has lived there。

这里has lived表示动作的持续性，时间的起点应从：“开始居住”时算起。因此此句可理解为“自从他（开始）住在那儿起，我就一直没收到他的来信”。

2）ince we have owned a car, we have gone camping every year。

自从我们自己有了汽车，我们每年都去野营。

试比较：（1）. 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。

Since I was at this school, they have had three headmasters.

自从我离开这所学校，他们已换了三个校长了。

（2）.He has never been to see me since I have been ill.

自从我生病以来，他从未来看过我。

 He has never been to see me since I was ill.

自从我病愈以来，他从未来看过我。

由此可见，since引导的从句持续动词的一般过去时和现在完成时所表示的意思恰好相反。换言之，前者以肯定的语法形式反映着否定的逻辑内容，而后者的形式与内容是一致的。

四、“It is +一段时间+since…”的句型与“It has been +一段时间+since…”意思相同。

前者是规范说法，后者是口语说法。应注意的是，在这种结构中，since引导的从句绝对不能用否定式的谓语动词来表示否定意义，下面两句都错了：

It is a long time since he didn’t study English.

It’s two months since you didn’t come to see me.

正确的说法为：

It’s a long time since he studied English

他好久没学英语了。

You haven’t come to see me for two months.

你有两个月没来看我了。

五、since和for

　Since 用来说明动作起始时间，for用来说明动作延续时间长度。
　I have lived here for more than twenty years.
　I have lived here since I was born..
　My aunt has worked in a clinic since 1949.　
　Some new oilfields have been opened up since 1976.
　I have known Xiao Li since she was a little girl.　
　My brother has been in the Youth League for two years.　
　I have not heard from my uncle for a long time.
　注意：并非有for 作为时间状语的句子都用现在完成时。
　I worked here for more than twenty years.
　(我现在已不在这里工作。)
　I have worked here for many years.　　　
　(现在我仍在这里工作。)
注：在“现在完成时+一段时间”这一结构中，我们用下面的公式转化，很容易就能排除非延续动词在完成时中的误使。
1)　(对) Tom has studied Russian for three years.
　　　 = Tom began to study Russian three years ago, and is still studying it now.
　2)　(错) Harry has got married for six years.
　　　 = Harry began to get married six years ago, and is still getting married now.
　显然，第二句不对，它应改为 Harry got married six years ago.　或 Harry has been married for six years.

六、as，since，because，for 
（1）because表示原因的语气最强，常表示必然的因果关系，从句是全句的重心，它引导的从句一般放在主句的后面；回答以why引起的特殊疑问句，只能用because。如：
He is absent today because he is ill．
他今天缺席，因为他病了。
—Why didn't he come？
—他为什么没来？
—Because he had an important meeting to attend．
—因为他有个重要的会议要参加。
（2）as只说明一般的因果关系，语气比because弱，说明比较明显的原因，它引导的从句通常放在句首，有时也放在句尾。如：
As everybody has arrived，now，we can set off．

既然大家都到了，我们可以动身了。
（3）since意为“既然”，表示对方已经知晓，无需加以说明的原因或事实（有时为天经地义的事实或格言、谚语等），语气比because稍弱，全句重点落在主句上。如：
I'll do it for you since you are busy．

既然你忙，我来替你做吧。
（4）for是并列连词，它的语气最弱，常用来补充说明理由或提供一种解释。它引导的句子不能放在句子的开头。如：
We must leave now，for the film begins at seven．

我们得走了，因为电影七点钟开始放映。