it's three years since i came to the school 中's代表的是什么的缩写

LS几位说的都对.楼主提到完成式,但要具体情况具体分析,英语中的例外不少：
since 是自从的意思.具体的句型构成是：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.
来自【英语牛团】

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