since的四種用法

1) since +過去一個時間點　(如具躰的年、月、日期、鍾點、1980, last month, half past six)。 I have been here since 1989.

2) since +一段時間+ ago I have been here since five months ago.

3) since +從句　 Great changes have taken place since you left. Considerable time has elapsed since we have been here.

4) It is +一段時間+ since從句 It is two years since I became a postgraduate student.