since,as,for,because引导的时间状语从句有什么不同

注意,since和as可以引导时间状语从句,for和because是不行的.
由since引导的时间状语从句.一般情况下,从句谓语动词用一般过去时,而主句的谓语动词用现在完成时.但在It is + 时间 + since从句的句型中,主句多用一般现在时（注意翻译）.例如：
I have been in Beijing since you left.自从你离开后,我一直在北京.
Where have you been since I last saw you?自上次我和你见面以后,你到哪里去了?
It is four years since my sister lived in Shanghai.我妹妹不在上海住有四年了.
as表示“一边……一边”,as引导的动作是延续性的,一般用于主句和从句动作同时发生；as也可以强调“一先一后”.例如：
We always sing as we walk.我们总是边走边唱.（as表示“一边……一边”）
As we was going out,it began to snow.当我们出门时,开始下雪了.（as强调句中两个动作紧接着先后发生,而不强调开始下雪的特定时间.）
这四个词都可以用来引导原因状语从句,差别如下
在语气上,because最重,表示的是直接原因,回答why时只能用它.其次是since,as,一般不表示原因,而是表明理由,进一步说明（译为：既然,由于）.For被认为是复合句的并列连词（常用于推断）,表示理由.如：
I didn’t go to school yesterday because I was ill.我昨天没去上学,因为我生病了.
Since everybody is here,let’s begin our meeting.既然大家都来了,让我们开始开会吧.
As you are in poor health,you should not stay up late.由于你身体不好,你就不该熬夜.
I asked her to stay to tea,for I had something to tell her.我请他留下来喝茶,因为我有事要告诉她.