【since】since用于什么时态

网友回答

【答案】 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.