since是一个从属连词，它可以引导时间状语从句｀原因状语从句和方式状语从句等，从句既可置于主句之前，亦可置于主句之后．
１．从属连词since引导时间状语从句，意谓＂自从......以来＂．since引导的时间状语从句有两种用法：
第一种是从句用现在完成时，从句用一般过去时，表过去某个时间开始直到现在．如：
How long has it been since you wrote to your sister?这句话的意思是:自从你上次写信给你妹妹,直到现在有多长时间了,强调的是过去你写信这个动作对现在的影响!
第二种是主句用过去完成时，从句用一般过去时或过去完成时，表过去某个时间知道过去某个时间．如：
（４）Since she came to me , we had been happy . 自从她来到我这里，我们一直很快活．
（５) Two weeks had passed since Martin had seen him . 自从马丁见到他以后，两周已经过去了．
since亦可用作介词．如：
（９）We hadn't seen each other since Oxford . 我们自从牛津别后就彼此没有见过面．
(10) I have not heard from him since writing last . 自上次写信以后，我没接到过他的信．
How long is it since you wrote to your sister? since做介词,意思与第二句差不多,但是强调的重点还有语气不同!这句强调的是现在!
How long ago did you write to your sister?
这句强调的是多久前,比较好理解!意思是:你是多久前写信给你妹妹的!
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