2017年12月英语六级语法常考考点：since的四种用法

英语自学网

　　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.