since作爲副詞形式一般用於現在完成時或過去完成時，作爲介詞形式一般用於現在完成時或過去完成時。
用法：
1、since+過去一個時間點（比如具體的年、月、日期、鐘點等）
例：I have been here since 1990.
2、since+一段時間+ago
例：I have been here since six months ago.
3、since+從句
例：Great changes have taken place since you left.
4、It is+一段時間+since從句
例：It is one years since I became a postgraduate student.
