Since引导的从句一般要用什么时态

一般过去时. 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 el