since 是自从的意思。具体的句型构成是时的词：主句现在完成时＋since一般过去时
但当主句的动词为be动词的 时候，还有三种特殊的用法：
It will be three years since I do the work.我从做这个工作起需要三年的时间（还没做呢）
It is three years since I did the work.我做这个工作到现在已经三年了。
It was three years since I had had done the work.我三年前做了这个工作。
since 引导时间状语从句，主句可以用各种时态；从句一般用过去时，有时也可以用完成时，此时从句谓语动词多为be或延续性动词如see, live, marry。
例：I can’t have seen him ～ 1988. 自从1988年以后，我不可能见过他。/It was years ～ I had seen her. 自从我见到她已经过了好多年。
since的四种用法
1) since +过去一个时间点 (如具体的年、月、日期、钟点、1980, last month, half past six)。
I have been here since 1989。
2) since +一段时间+ ago
I have been here since five months ago。
3) since +从句
Great changes have taken place since you left。
Considerable time has elapsed since we have been here。
4) It is +一段时间+ since从句
It is two years since I became a postgraduate student。
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是since吧，的确是现在完成时的标志，如果要用在过去的话，只能用在过去完成式