［動詞的時態(tài)］since的四種用法
1) since +過去一個時間點(如具體的年、月、日期、鐘點、1980， last month， half past six)。例如：
I have been here since 1989. 1989起，我一直在這兒。
2) since +一段時間+ ago.例如：
I have been here since five months ago. 我在這兒，已經有五個月了。
3) since +從句。例如：
Great changes have taken place since you left. 你走后，變化可大了。
Great changes have taken place since we were here. 我們走后，變化可大了。
4) It is +一段時間+ since從句。例如：
It is two years since I became a postgraduate student. 我考上研究生有兩年了。
