[2]It is three years since he was a teacher.= Three years ago, he was a teacher. (他不当老师三年了。)（三年前他还是老师，现在不是了。）

　　[3]It is two years since my father smoked.=Two years ago, my father to smoke. (我父亲不吸烟已有两年了。)

　　[4]It is two years since my father began to smoke.= Two years ago, my father began to smoke.(我父亲吸烟已有两年了。)

　　5】 It/This is the first/second …time that+主语+have/has done…(从句一定用完成时。) 这是某人第。。。次做了某事。

　　[1] It is the second time that she has written to her mother.

　　[2]This was the first time that I had come late to school.

　　【6】It was+时间点+when-从句

　　It was 10 o’clock when my father got home last night.

　　【7】 It was/will be+some time+before-从句（注意主从句的时态应前后对应）。此句型表示：过了/还有多少时间才/就。。。。。。

　　[1] It was seven years before they finally got married.（主句用一般过去时，从句用一般过去时。）过了七年他俩才结婚。 [2] It won’t be long before we graduate from No.1 Middle School. （主句用一般将来时，从句用一般现在时。）不久，我们就要从一中毕业了。

　　it作形式主语，代替由不定式、动名词或从句所表示的真正主语。

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