现在完成时的标志词,sice不是现在完成时的标志吗？

since 是自从的意思。具体的句型构成是现在完成时的标志词：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。

。

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

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