since
单词辨析：
一般不与现在时间发生关系，故常和动词的过去时或过去完成时连用。
After school he went home.
放学以后，他回家去了。
She began her study after moving to London.
搬到伦敦后她开始了自己的学业。
He came to see me after he had arrived.
他到达后就来看我了。
〔辨析、例证〕
一般包含现在时间，故常和动词的一般时或完成时连用。
1、主句中动词用现在完成时，since 引导的从句中动词用一般过去时。
You've changed a lot since we last met.
自从上次我们见面以来，你变化好大啊。
Great changes have taken place in Beijing since 1985.
自1985 年以来北京发生了很大的变化。
2、主句中主语为 it，动词用一般现在时或一般过去时，since 引导的从句中动词须用一般过去时或过去完成时。
It is two years since we last parted.
我们上次分手以来已有两年了。
It was two years since we had last parted.
那时我们已经分手两年了。
语气最强，表示因果关系，通常用来回答由 why 引导的疑问句。由 because 引导的分句置于句首时，须用逗号与主句隔开，置于句末时则不用隔开。
Because the traffic is so busy, she failed to arrive at the station on time.
因为路上交通太拥挤，她没能准时赶到车站。
He came back late because there was something wrong with his car.
他回来晚了，因为汽车出了故障。
如果所述理由是明显的或者已知的，则用 as 较 because 更为合适。
As it's raining, you'd better stay at home.
既然在下雨，你最好呆在家里。
He puts the ice cream into the fridge immediately, as it melts easily.
他马上把冰激凌放进冰箱，因为它很容易化掉。
for 属于并列连词，由它引导的分句表示补充说明性的理由，通常只能置于句末，且必须用逗号与前面的分句隔开。
He must have passed this way, for here are his footprints.
他肯定是从这条路走过去的，这里有他的脚印。
He has to work hard, for he wants to support his family.
因为要养家糊口，他必须努力工作。
当所述原因是已知的客观事实或被当作推断的理由时用 since，语气略强于 as。
Since he is absent, let's begin.
既然他不在，我们就开始吧。
Susan must have come back home, since her coat is not in the wardrobe.
苏珊肯定回过家，因为她挂在衣橱里的外套不见了。
相关例句：
Jacques Arnold has been a member of parliament since 1987
雅克·阿诺德自1987年以来一直是议会议员。
It's only a year since I last saw the child and he's grown so tall.
这孩子一年不见，长这么高了。
The period since the revolution has been one of political turmoil.
革命爆发后就陷入了政治动乱期。
Prices have climbed by 21% since the beginning of the year
年初以来，物价已经上涨了21%。
I have long since come to terms with my blindness.
我早就安于自己的失明了。
It was some weeks now since they had had any respite from shellfire.
他们已经好几周没有在炮火中获得任何喘息的机会了。