都用完成时，完成时包括许多
since
adv.(常用于现在完成时或过去完成时)
1. 此后;从那时到现在
She left school three years ago and has worked as a nurse ever since.
她三年前毕业,从那时起便一直当护士。
2. 之前,以前
prep.
1. (常用于现在完成时或过去完成时)自...以来,从...至今
I haven't heard from him since last year.
我自去年以来未曾收到过他的信。
conj.
1. (常用于现在完成时或过去完成时)自...以来,从...至今
It's been ten years since they married.
他们结婚到现在已经十年了。
She has been living a hard life since her husband died.
她自从丈夫死后,一直过着艰苦的生活。
2. 既然;因为,由于
He must have taken the book since it isn't here.
他一定是把书拿走了,因为书已不在这里了。
Since it is so hot, let's go swimming.
既然天气这么热,我们去游泳吧。
since的用法主要有两点：一是表示“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作连词引导时间状语从句的用法。
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：
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。
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：
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”。
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：
How long is it since you lived in Shanghai?你离开（没住在）上海多久了？
It is half a month since he was a League member.他退团（不当团员）半个月了。
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：
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”。
Since 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。
至于It's+一段时间+since…的句型与It has been +一段时间+since…意思相同。前者是口语说法，后者是规范说法。
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