since用于什么时态？(since用于什么时态中)

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.