since引导原因状语从句,相当于now that,意思是“因为……,既然……,鉴于……”.
Since the rain has stopped,let’s go for a walk.
既然雨停了,我们出去散散步吧.
Since we are young,we shouldn’t be too afraid of making mistakes.
因为我们还年轻,所以不应该太害怕犯错误.
注意：since表示原因时,语气比because弱,表示的原因是对方已知的,或是句中不很重要的部分,是附带的原因,因此引导的从句不是信息的焦点,不能回答why的提问,也不能用在强调句型中作为被强调的内容.since引导的从句往往放在主句之前.
2.引导时间状语从句,此用法较复杂.
(1) 若since引导的状语从句的谓语动词是终止性动词(如go,come,leave,start,begin等)的过去时,则从句表示的时间是“从该动作开始的那一时刻起”.这是最常见的一种用法.如：
He has studied very hard since he came to our school.
自从来到我们学校,他学习就非常努力.
Great changes have taken place in our school since you left.
自从你离开这里,我们学校发生了很大的变化.
(2) 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静态动词(如live,stay,study,learn,smoke,be等)的过去时,则从句表示的时间是“从该动作或状态的完成或结束时算起”.如：
I haven’t heard from him since he lived here.
这句话应译为：从他不住这儿起,我一直没有收到他的信.
I haven’t eaten snakes since I was a student at school.
was表示状态,因此该句可理解为：I haven’t eaten snakes since I left school.
自从我毕业后就一直没吃过蛇.
试比较：
He has written to me frequently since I was ill.
自从我病愈以来,他经常给我写信.(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,他经常给我写信.(从句谓语为终止性动词)
(3) 若since引导的状语从句的谓语为持续性动词或静态动词的现在完成时,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：
He has written to me frequently since I have been ill.
这里have been ill表示状态的持续性,时间的起点应从：“开始生病”时算起.因此此句可理解为：“自从我生病以来,他经常给我写信.”
试比较：
I’ve written her 20 letters since I have been here.
自从我来到这,我已经给她写了20封信.(从开始算起)
I’ve written her 20 letters since I was here.
自从我离开这儿,我已经给她写了20封信.(从结束算起)
(4) It is + 段时间 + since.../ It has been + 段时间 + since...表示“从……起已有多长时间了”.两句型意思相同,前者是规范说法,后者是口语说法.
用法：since从句中常用一般过去时,若从句中是持续性动词的一般过去时,则表示该动作结束有多长时间了.如：
It is three years since he smoked.
他不吸烟已有3年了.
试比较：
It is three years since he began to smoke.
It is three years since he has smoked.两句均意为：他吸烟已有3年了.
注意：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,试比较：
他好久没学汉语了.
误：It is a long time since be didn’t study Chinese.
正：It’s a long time since he studied Chinese.
(studied为持续性动词,应从其动作结束时算起)