由since、for引导的时间状语从句

关于since

1. 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. 
We have been missing them since they left here. 
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动词的过去式时，则从 
句表示的时间是“从那持续动作或状态结束时算起”。如： 
How long is it since you lived in Shanghai? 
John is now with his parents in New York. It is already three years since he was a teacher.

关于for

(1) for是并列连词(其作三者为从属连词)，它有时可表示因果关系(只能放在主句之后，且可与because换用)， 

有时不表示因果关系，而是对前面分句的内容加以解释或推断(也只能放在主句之后，但不能与because换用)。 

The ground is wet, for (或because) it rained last night. 地面是湿的，因为昨晚下过雨。 

It rained last night, for (不能用because) the ground is wet this morning. 昨晚下过雨，(你看)今天早上地面都是湿的。 

(2) for “因为,由于”表明附加或推断的理由，引导的分句前常有逗号。for分句不放句首.是用来附带解释或说明前一分句的原因和理由。 

Day breaks, for the cock crows. 

It must have rained last night, for the ground is wet. 

I won't go in, for my father is there.