since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）。

　　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。