小学英语语法知识点：since的四种用法

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小学英语语法知识点：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.